Help wanted: Continuous integration

It would be great to get a PR helping us setup a CI build on Travis. This would also force us to iron out our build on Linux. It's not as simple as just slapping a travis.yml file into the root folder however...

Running only the tests that matter

We're planning on xray being a mono-repository to avoid the transaction costs and community fragmentation of hosting hundreds of separate, interconnected repos. However, it seems like this could make our builds horrible if we're not careful.

For this reason, even though we're versioning all these components together, we want their inter-dependencies to be clearly specified, and we want the global build script to be smart about knowing when to run which component's test suite.

So for example, if we make a change in xray_core, then we want to run tests on xray_node and xray_electron. A change to xray_electron should run tests only on xray_electron. A change to the documentation should run no tests.

How should we achieve this? We could cache some sort of fingerprint file on a per-module basis with a digest of the module's contents, plus make each module's build smart about checking the fingerprints of dependencies. Or maybe there's a better, standard way of dealing with this in mono-repos?