Enhance Testing With Docker

Ideally it would be nice to test against a local instance of Gerrit. This should provide several new benefits for the project and its contributors:
- Testing can be performed without depending on a remote instance of Gerrit. This isolates go-gerrit from upgrades, infrastructure changes and outages while also preventing tests from impacting someone else's Gerrit installation.
- Setup/teardown steps could include creation of real changes, accounts, projects etc. This would mostly remove the need to mock responses within the tests.
- It would provide a means for contributors to test against and identical installation of Gerrit for every test run.
- Test can be written without having to worry about permission issues or the possibility of modifying production data.
- In more advanced use cases, this setup could be used to test against specific configurations of Gerrit.

As part of this enhancement, the following modifications would be made to the code:
- All tests would point at a configurable URL.
- Docker-compose would be used on Travis to run Gerrit behind nginx. This will be more production like than just running Gerrit by itself and it also simplifies 'running gerrit' to a single command.