Continuous integration and test coverage

@alainjobart @sougou @enisoc @yaoshengzhe 

Integrated with Travis and Coveralls
- https://travis-ci.org/youtube/vitess
- https://coveralls.io/r/youtube/vitess

Integrations tests are grouped into 3 suites and this helps with parallelization of the builds.
All targets are retried up to 3 times before they get marked as failed. 
Skips a few tests that are too flaky even after the retries.
Have turned off email notifications for now, will turn this back on once merged into master.