Make source jars from java_library available to Skylark.

[ekuefler]

This is necessary to implement GWT support in Bazel, which requires source jars
on the classpath. With this change, I can build a GWT classpath as follows:

  all_deps = ctx.files.deps
  for this_dep in ctx.attr.deps:
    p = provider(this_dep, "rules.java.JavaSourceJarsProvider")
    if p: all_deps += p.source_jars
  classpath = ":".join([dep.path for dep in all_deps])

See #146 for discussion

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[damienmg]

IMO Exposings this provider makes sense but can we have documentation? Also if you use your @google.com email when sending a PR then the googlebot won't complain. (Btw, for real review, we prefer to use Gerrit at https://bazel-review.googlesource.com).

[ekuefler]

No problem, added docs and pushed to Gerrit at #214. I'm not at Google any more, so I'll have to stick with this email, but I've signed the CLA now.