Crash if server returns reserved CONNACK message

[Decrypto]

Background:
AdafruitIO has extended the MQTT protocol by adding two additional CONNACK return codes:

6: Connection Refused, throttled
7: Connection Refused, banned

https://io.adafruit.com/blog/mqtt/2016/06/28/extending-the-mqtt-protocol/

Under normal circumstances, these codes shouldn't come up. However, I have engineered my code to reconnect after a slight delay on disconnection. Due to #98 this code currently is being triggered frequently, leading to AdafruitIO throttling further connections.

Issue:
When the server returns a CONNACK message with a code other than the formally defined 0-5 (
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718033 ), CocoaMQTT crashes in CocoaMQTT.didReceiveConnAck(reader:, connack:) at:

let ack = CocoaMQTTConnAck(rawValue: connack)!

The initializer for CocoaMQTTConnAck returns nil when passed a rawValue outside 0...5.

`fatal error: unexpectedly found nil while unwrapping an Optional value
2016-10-20 17:12:14.708461 Ahem[1782:42159] fatal error: unexpectedly found nil while unwrapping an Optional value
Current stack trace:
0    libswiftCore.dylib                 0x0000000100458580 swift_reportError + 132
1    libswiftCore.dylib                 0x0000000100475850 _swift_stdlib_reportFatalError + 61
2    libswiftCore.dylib                 0x0000000100271550 specialized specialized StaticString.withUTF8Buffer ((UnsafeBufferPointer) -> A) -> A + 355
3    libswiftCore.dylib                 0x00000001003e9600 partial apply for (_fatalErrorMessage(StaticString, StaticString, StaticString, UInt, flags : UInt32) -> Never).(closure #2) + 109
4    libswiftCore.dylib                 0x0000000100271550 specialized specialized StaticString.withUTF8Buffer ((UnsafeBufferPointer) -> A) -> A + 355
5    libswiftCore.dylib                 0x00000001003a4970 specialized _fatalErrorMessage(StaticString, StaticString, StaticString, UInt, flags : UInt32) -> Never + 96
6    CocoaMQTT                          0x00000001001b64e0 CocoaMQTT.didReceiveConnAck(CocoaMQTTReader, connack : UInt8) -> () + 149
7    CocoaMQTT                          0x00000001001b86d0 protocol witness for CocoaMQTTReaderDelegate.didReceiveConnAck(CocoaMQTTReader, connack : UInt8) -> () in conformance CocoaMQTT + 76
8    CocoaMQTT                          0x00000001001ba7c0 CocoaMQTTReader.frameReady() -> () + 440
9    CocoaMQTT                          0x00000001001ba220 CocoaMQTTReader.payloadReady(Data) -> () + 1403
10   CocoaMQTT                          0x00000001001b5250 CocoaMQTT.socket(GCDAsyncSocket, didRead : Data, withTag : Int) -> () + 4089
11   CocoaMQTT                          0x00000001001b62b0 @objc CocoaMQTT.socket(GCDAsyncSocket, didRead : Data, withTag : Int) -> () + 95
12   CocoaAsyncSocket                   0x0000000100133900 __37-[GCDAsyncSocket completeCurrentRead]_block_invoke + 83
13   libdispatch.dylib                  0x0000000100aac741 _dispatch_call_block_and_release + 12
14   libdispatch.dylib                  0x0000000100aa2fc4 _dispatch_client_callout + 8
15   libdispatch.dylib                  0x0000000100ab1a71 _dispatch_main_queue_callback_4CF + 362
16   CoreFoundation                     0x00007fff9eca9c80 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
17   CoreFoundation                     0x00007fff9ec6a810 __CFRunLoopRun + 2205
18   CoreFoundation                     0x00007fff9ec6a410 CFRunLoopRunSpecific + 420
19   HIToolbox                          0x00007fff9e20be7c RunCurrentEventLoopInMode + 240
20   HIToolbox                          0x00007fff9e20bbf1 ReceiveNextEventCommon + 432
21   HIToolbox                          0x00007fff9e20bb8f _BlockUntilNextEventMatchingListInModeWithFilter + 71
22   AppKit                             0x00007fff9c9021b0 _DPSNextEvent + 1093
23   AppKit                             0x00007fff9d012286 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1637
24   AppKit                             0x00007fff9c8f6c1f -[NSApplication run] + 926
25   AppKit                             0x00007fff9c8c15b5 NSApplicationMain + 1237
26   Ahem                               0x0000000100027750 main + 84
27   libdyld.dylib                      0x00007fffb3caa254 start + 1
`

It would be nice (and more future-safe as MQTT evolves) if CocoaMQTT could handle reserved values more gracefully. There may be other instances of this class of issue besides CONNACK.

[CrazyWisdom]

@Decrypto
#109