lookout-gometalint-analyzer implementation

[bzz]

Initial implementation of test analyzer that wraps gometalint and fixes https://github.com/src-d/lookout/issues/136

• _fixtures are used in tests to trigger specific comments
• util/grpchelper is a copy of lookout (due to dependencies on grpc that both projects /vendor/)

the goal is not to have a production-ready analyzer, but rather to gain experience writing analyzers before publishing SDK and to have something operational on staging.

TODOs:

• [x] build, CI, docs, etc
• [x] analyzer impl
• [x] k8s service

[bzz]

\cc @carlosms for a review

[bzz]

history re-written to update k8s descriptor