Build improvements

@capitancambio I couldn't help tinkering with the build setup and came up with the following approach:

- introduced a Makefile to build stuff locally and with Travis (`GOPATH` is set to a git-ignored `build` directory).
- use Maven for deployment, producing a ZIP assembly in `target` for each sub-directories produced in `build/bin`
- adapted the Travis config
   - only build `master` or PRs on `master`
   - `make` s/b invoked by default, i.e. `go test && go build`
   - if it succeeds, run and deploy test coverage, build binaries for x-platform, and deploy with Maven

The coverage part is not tested, but the Maven deploy works locally, and should work in this PR.