Event raising and semi-asynchronous activity

[gbakeman]

Make the library thread-aware and implement an Event-raising system to raise events for common things, such as:

Connection monitoring (see issue #24)
Connection established/disconnected
Variable data updated (asynchronous variable retrieval)

• HandleServerConnected
• HandleServerDisconnected
• HandleNUTException
• HandleConnectionBroken

[gbakeman]

I'll make as much of the socket asynchronous as possible, although NUT servers (upsd) operate synchronously so there's not much point in async sending/receiving.