Add possibility to easily set LibGDX version

Developing on nightlies is good for bleeding edge projects. For some projects
we might want to use more stable environment. Added two variables that allows
us to set LibGDX version during project creation. By default, we still use
latest nightlies.
Fix proguard.cfg template generation

I've skipped obvious issue... template for proguard.cfg wasn't generating
correctly because I forgot to escape dollar signs. Sorry about that. Anyway,
this commit fixes it.
Merge pull request #20 from aginiewicz/proguard-fix

Fix proguard.cfg template generation
Merge pull request #21 from aginiewicz/libgdx-version

Add possibility to easily set LibGDX version