Add 'FlxRandom' pseudo-random number generator

The new `FlxRandom` class contains the following methods:

```as3
float(min:Number = 0,max:Number = 1):Number
boolean(chance:Number = 0.5):Boolean
sign(chance:Number = 0.5):int // Returns either 1 or -1
bit(chance:Number = 0.5):int // Returns either 1 or 0
integer(min:Number, max:Number = NaN):int
item(array:Array, startIndex:uint = 0, length:uint = 0):* // Pick a random item out of an array between 'startIndex' and 'length'
shuffle(array:Array, modifyArray:Boolean = true):Array
```

Any methods or properties that are replaced by this method get their own little `@deprecated` tag in the ASDoc, as well as a warning written to the console every time they try to use that method.