Reworking of how sprites are flipped, closes #659

* Removed "flippable"
* Added ability to flip sprites both horizontally and vertically
* Flipping based on facing direction is now customizable
* Added FlxAxis enum

Previously, in order to make a sprite flip horizontally when it faces left, you would put:
```
    flippable = true;
```
This has now been replaced with:
```
    setFacingFlip(FlxObject.LEFT, FlxAxis.X);
```
This now allows for combination directions, using bitwise OR for union of directions. For example:
```
    setFacingFlip(FlxObject.LEFT | FlxObject.UP, FlxAxis.X); //Flip horizontally if facing up and left
```