Setup core and pc modules to build with gradle.
Add snakegame lib and pc to gradle build.
Centralize maven repo config' for gradle build.
Add sxeTarget property to gradle build.

This works like the maven profiles, basically build core by default, and
have a way to specify pc or android build. Android doesn't have gradle
setup yet!
Change output jars to sxe-{core,pc} in gradle build.
snakgame/pc now uses application plugin for gradle build.
setup extract of LWJGL natives
s/null/''/
Finish up native libraries support for gradle build.
Added TODO and README file.