automatically export //java:grpc_compiletime_deps when with_Grpc = true

[perezd]

This makes it easier on users to not have to remember to import //java:grpc_compiletime_deps on their own.

[perezd]

This addresses #52.

[pcj]

The tests have been a little flaky lately, and travis queue seems to be getting longer and longer. I restarted the job.

[pcj]

Can you:

1. Document this behavior in the README.md for the java rules?

2. Comment out the grpc_compiletime_deps in the client and server helloworld example, leaving a comment that it is automatically exported by the proto rule?

[perezd]

OK, updated those items.

[perezd]

Oh shoot, forgot the comment in the build file...should I add that still?

[pcj]

Might be nice in case someone has a non-first order dependency that won't be satisfied by export, less digging to figure out the magic.

If you want to just commit --amend and git push --force auto-export-grpc won't make an extra commit.

[perezd]

OK all patched up.

[pcj]

LGTM