Move photonstorm classes to more appropriate places

This is a preparation for #461.

This is a controversial change for sure. @Beeblerox and @impaler seem to be mostly worried that the removal of the photonstorm package this includes by moving the classes elsewhere removes most of the credit from @photonstorm - credit that he totally deserves for creating this awesome set of classes!

I think the change is for the better though, it makes the planned `flixel-addons` repo more worthwhile by adding more classes to it. Also, the packages and the function of the classes within them now actually fit together - previously, the power tools classes were contained in the `plugin` package, even though `FlxMouseControl` is the only actual flixel plugin (afaik)?

This change allows these classes to evolve by integrating them into other parts of the engine, now that the power tools are not being updated anymore.

Here are the changes in detail:

- `plugin.photonstorm.FlxColission` -> `flixel.util.FlxColission`
- added a `pixelPerfectOverlap()` function to `FlxG` which is a shortcut for `FlxColission.pixelPerfectCheck()`

- merged `plugin.photonstorm.FlxCoreUtils` into `flixel.util.FlxMisc`

- `plugin.photonstorm.api` -> `addons.api` 
- `plugin.photonstorm.FlxWeapon.api` -> `addons.weapon.FlxWeapon` 
- `plugin.photonstorm.baseTypes.FlxBullet` -> `addons.weapon.FlxBullet` 
- `plugin.photonstorm.FlxExtendedSprite` -> `addons.display.FlxExtendedSprite` 
- `plugin.photonstorm.baseTypes.FlxMouseSpring` -> `addons.display.FlxMouseSpring` 
- `plugin.photonstorm.FlxMouseControl` -> `addons.plugin.FlxMouseControl` 
- `plugin.photonstorm.FlxBitmapFont` -> `addons.text.FlxBitmapFont` 
- `plugin.photonstorm.FlxDelay` -> `addons.util.FlxDelay` 
- `plugin.photonstorm.FlxControl` -> `addons.control.FlxControl` 
- `plugin.photonstorm.FlxControlHandler` -> `addons.control.FlxControlHandler` 
- `plugin.photonstorm.FlxGridOverlay` -> `addons.display.FlxGridOverlay` 

Opinions?