Adding the java language level to the IdeaGen module output.
Review Request #2911 - Created Sept. 30, 2015 and submitted
Calculating the java languagel level as the maximum source level over all jvm targets in the project. Ideally we'd like to specify a different language level on a per-directory basis, but this is helpful for now.
Added test cases to idea_integration. Works as intended with manual testing in our internal repo.
CI went green here: https://travis-ci.org/gmalmquist/pants/builds/82995933