add check to make sure the required android.jar exists
skip java task if source directory is missing
A few dependency-related changes

- Better handling when manually specifying retrolambda version
- Don't include javaagent when not needed
- Bump default retrolambda version to 1.6.0
new method of incremental compilation that doesn't break all the things
simplify dependencies a bit
version bump and README updates
update android studio instructions
Merge branch 'develop'

Conflicts:
	README.md
	build.gradle