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
Fix markdown formatting in CONTRIBUTING
Spreading the content over sections

I spread the content over sections to make it easier on the eyes. I also added a "How you can help" section at the beginning, it should guide newcomers.
Enhance the README a little bit

I've created some sections to the README file to make it easier to read. I've spread the content the better way I could to make a newcomer understand what Flixel Community is, reading just a few words. Veterans can understand the "community thing" scrolling down just a bit.
Merge pull request #153 from IQAndreas/fix-destroy

Prevent errors if `destroy()` is called twice
Merge pull request #2 from Dovyski/patch-1

Spread the content over sections
Merge pull request #3 from Dovyski/patch-2

Enhance the README a little bit
Tiny `README` typo
Change LICENSE to point to list of contributors on GitHub
Merge pull request #155 from IQAndreas/readme-markdown

Make repository documentation into Markdown
Merge pull request #3 from FlixelCommunity/dev

Update personal /dev branch to track Flixel-community changes
Merge branch 'dev' into dark-flixel

Conflicts:
	org/flixel/system/FlxQuadTree.as
	org/flixel/system/debug/VCR.as
	org/flixel/system/debug/Vis.as
	readme.txt