Add `FlxRandom` class

Basic framework set up; more to come!
Add basic methods to 'FlxRandom'

Also added a slight notice at the top crediting the sources for most of
the code.
Add more features to 'FlxRandom'

Also added more arguments to some of the existing functions.

I think I'm almost satisfied with this class now.
Improve `FlxRandom` ASDoc

Plus some slight bug fixing (also removed two "untyped" warnings that I
missed). Pretty good turnout for a class I didn't test before
committing.
Deprecate old methods involving randomness

I didn't delete them, just moved them "out of the way" and added a
`@deprecated` flag, as well as a warning every time someone tries to
access them (see `FlxG.warnDeprecated()`).
Update existing references to deprecated methods to point to new members

Also did some performance tweaking, and in some cases, cut out large
chunks simply for the sake of clarity, nothing more.
Fix ASDoc for FlxRandom

Apparently, you can't have anything after the @see (or any other
@something) variables.

Second, the ASDoc compiler doesn't like the less than or greater than
symbols.
Merge pull request #207 from IQAndreas/flx-random

Add 'FlxRandom' pseudo-random number generator