Add testing of Bigtable support via Bigtable emulator

PR https://github.com/JanusGraph/janusgraph/issues/103 added support for Google Cloud Bigtable via the HBase backend, but the tests are only being run using HBase. We should add tests to run against the [Bigtable emulator](https://cloud.google.com/bigtable/docs/emulator).

@lindsayismith, I believe you ran the HBase test suite manually with the Bigtable emulator when you made the changes in PR #103; would you mind adding an automated way to run it so that they run every time with Travis CI to ensure that everything continues to work properly with the Bigtable backend?

It would also be good to verify that any new changes, as well as new client library versions, continue to work well with JanusGraph + Bigtable, and this would be the way to do it.

Start with the existing HBase test suite and the Travis CI configuration; the issue does not name specific files. Add an automated run against the Bigtable emulator so the suite executes in Travis CI, and verify that the Bigtable backend continues to work with new changes and client library versions.

Written by the indexing model from the issue text.