Added jaxb support for pants.

jaxb_library target now exists. It takes in sources=[] for the schema file(s), and a package='...' parameter for what package to generate the output java files in. It uses the xjc compiler included in the jdk.
jaxb_library target now exists. It takes in sources=[] for the schema file(s), and a package='...' parameter for what package to generate the output java files in. It uses the xjc compiler included in the jdk (executed using n

Testing Done:
Python unit test under tests/python/pants_test/tasks/test_jaxb_gen.py - testing correct filename and package generation
Java unit test under tests/java/com/pants/examples/usejaxb - testing jaxb_library as a dependency in java code which uses generated classes

Jaxb file generation and compilation worked successfully for arbitrary package names, with an arbitrary number of output source files. XML class instance was read in against an .xsd schema, and properly filled fields in the ge

Reviewed at https://rbcommons.com/s/twitter/r/446/