Added Jaxb support for pants.

JaxbLibrary and JaxbGen were modeled after code for antlr, protobuf, and thrift code generation. .xsd schema files are compiled with the 'xjc' binary included in the java jdk. This changes adds a jaxb_library target to compile schema files. Besides standard arguments (eg, sources), jaxb_librarys takes in a 'package' argument which defines the output package for generated java files.