FlxTilemap ignores last tiles if startingIndex is > 0

_tileObjects now takes startingIndex into account.

Fixed FlixelCommunity/flixel#55, AdamAtomic/flixel#176
overlapProcessed isn't properly cleared in FlxQuadTree

_notifyCallback is now only called is _processingCallback returned true that iteration.

Fixed FlixelCommunity/flixel#29, AdamAtomic/flixel#203
FlxTilemap not clearing variables before loadMap

I've added a clearTilemap function that properly resets everything before every call to loadMap. I also set visible and active to false in the constructor, so that flixel doesn't attempt to draw a tilemap before it's been loaded.

Fixed FlixelCommunity/flixel#46, AdamAtomic/flixel#185
Fixed FlixelCommunity/flixel#34, AdamAtomic/flixel#197
Fix FlxObject#(overlaps|overlapsAt) checking only first element of FlxGroup

FlxObject#overlaps and FlxObject#overlapsAt had a bug that would lead to only the first element of a FlxGroup being checked. It was happening because the variable "length" was not locally declared, however there was no compiling warns/errors because the Object class has a property named "length".
Add `FlxGroup::revive()`

The FlxGroup will now revive itself, and then all its members.

This is to compliment `FlxGroup::kill()`.
Tweaks for FlxObject.overlap* and fixes for FlxTilemap.overlap*

I extended the fixes of FlxObject.overlap*() to FlxTilemap.overlap*(). I also added a break statement when a collision is detected since all overlap() methods return just a boolean.

Fix https://github.com/FlixelCommunity/flixel/issues/39 and https://github.com/AdamAtomic/flixel/issues/192 .
Small code refactoring.
Fix minor typos in `FlxGroup::revive()`

It's a minor typo, but with major consequences!

If left unchecked, this bug would revive already existing objects,
leading to several Science Fiction worthy paradoxes.

Also fixed a tiny spelling typo in the ASDoc.
Only revive `FlxGroup` objects if they are `alive`

After some discussion on GitHub
https://github.com/FlixelCommunity/flixel/pull/110/files#r1831839
it seems the more logical to check if an object is `alive`,
rather than check if it `exists` when reviving it.
Fix `FlxCamera` jitters when following a target

Resolves the following issue:
 * https://github.com/FlixelCommunity/flixel/issues/5 - https://github.com/AdamAtomic/flixel/issues/227

Fix provided by [Krix](https://github.com/krix)
Rename `FlxSprite::simpleRender` to `isSimpleRender`

Will also avoid an error if the `target` object is not a `FlxSprite`.
no need to fill a camera twice if not blending alpha
Prevent errors if `destroy()` is called twice

Add checks before accessing object properties to see if they hadn't already been set to `null`.

Not modified the package `org.flixel.system.debug.*`
Prevent `destroy()` errors in `org.flixel.system.debug.*`

Now all classes check if an instance is `null` before accessing its members.
Make LICENSE and README files into Markdown files

Previously `.txt` files
Update LICENSE and README formatting to take advantate of Markdown

They are prettier now. :)
Add CONTRIBUTING file

The file will help guide new GitHub users who want to contribute to Flixel.

See the following blog post for more information:
 - https://github.com/blog/1184-contributing-guidelines
Prevent errors if `VCR` or `Vis` are missing the parent

The way `FlxDebugger` is set up, the `parent` will ALWAYS be `null`,
since the instance is removed before as a child before being destroyed.

This is only a  (:::[ ]:::)
Add 'FlixelCommunity Team as' additional copyright holder
Add FlixelCommunity information to the README