Add tests

This should be enough to serve as a basis for discussion.

I replaced `walkdir` with `glob`, added first (very simple) tests, activated travis ci builds, and added badges to the readme.
Tests pass on travis ci and on a local windows vm.

I'm not sure if it's necessary/desired to test API and CLI seperate, but currently that's what I do.

Added dependencies: `glob`, `mkdirp`
Added devDependencies: `mocha`

If you were to merge this, you would have to activate `atom/asar` in travis ci.

What's missing?
- More tests obviously. (But feedback first.)
- Appveyor windows builds. (I have to read a little bit about it first.)

I will compare the speed between the `walkdir` and the `glob` version, but haven't done it yet.

So, feedback? Questions? Change requests?

Have a nice day.