use coveralls for test coverage

[tstack]

I tried to add test coverage on my fork, but the support for rust is still a bit lacking. I tried to use https://github.com/roblabla/cargo-travis, but there is an issue with more recent versions of cargo. I tried to use an unreleased branch of cargo-travis, but the build failed with some linking errors. So, I switched to https://github.com/xd009642/tarpaulin, but that doesn't work for CLI integration tests. I think I'm going to give up at this point and just wait for tarpaulin's support to improve. Just wanted to file this issue to log what was done so far.