Lib folder is an anti pattern

Moved all pieces of lint-trap to root path and eliminated `./lib/` since it is supposed to be a staging area for modules that should eventually be installed into the `./node_modules/` folder. 

Also renamed `./lib/index.js` to `./lint.js` since it is best if all parts of the module are named according to their function.

This addresses code review issues here:
https://github.com/uber/lint-trap/pull/14

@Raynos