Add DynamicArray feature.

[inamiy]

This pull request will add DynamicArray and ForwardingDynamicArray classes
which uses KVC's mutableArrayValueForKey() magic to interact with existing NSArray models.

Example

DynamicArray

let dynamicArray = DynamicArray()

// REACT
dynamicArray.signal ~> { changedValues, change, indexSet in
    println(change)
}

// add object to proxy (NSMutableArray)
dynamicArray.proxy.addObject(123)

// `change = .Insertion` will be printed

ForwardingDynamicArray

let model = MyModel(array: [1])

// pass NSMutableArray so that `ForwardingDynamicArray` can tweak it
let dynamicArray = ForwardingDynamicArray(model.mutableArrayValueForKey("array"))

// REACT
dynamicArray.signal ~> { changedValues, change, indexSet in
    println(change)
}

// add object to proxy (NSMutableArray)
dynamicArray.proxy.addObject(2)

// `change = .Insertion` will be printed & `model.array` will be updated as `[1, 2]`

For XCTest, please see ArrayKVOTests.swift#L224-L369.

For UI example, please see ReactKit/ReactKitCatalog#6.

[inamiy]

TODO: needs fix

• Create a new signal for each dynamicArray.signal call rather than reusing it
• Fix retain cycle: ForwardingDynamicArray -> forwardingSignal -> KVOProxy -> ForwardingDynamicArray (boldface must be avoided)