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.
So I tried this out but unfortunately couldn't get it to work. Issue is go1.3 doesn't install golang.org/x packages to GOTOOLDIR but instead treats it as a regular package and installs to GOPATH. This is troublesome because the 'go tool' command only looks in GOTOOLDIR. I tried bumping up the version to v1.4 but our tests do not pass in 1.4. There are some issues with zkctl shutdown behavior which I have not investigated thoroughly. 
https://travis-ci.org/youtube/vitess/jobs/44167939
Until we are confident about 1.4, I think we should continue to use code.google.com version of cover.