Create a skylark repository rule for maven artifacts

[kchodorow]

Some FRs that have come up in the past:

• Support dependency management.
• Download src and doc jars.
• Allow cloning from local repositories.
• Transitive deps?

[kchodorow]

• Support sha256 as well as sha1 (http_file uses sha256 but maven_jar uses sha1 #991).
• Support testonly (maven_jar should allow a testonly attribute #1407), should come for free.

[jart]

Added a proposal sorta related to this in #1733. Feel free to close it out and schlep it into this issue.