Add Editor Config + JSHint + Travis

Related to #11 

I worked a bit on it. I added .editorconfig, developer must use a [plugin for the IDE](http://editorconfig.org/#download).

With JSHint was hard, a lot of unused variable that I'm not sure if I can remove, so, I've set [undef](https://github.com/felquis/BackstopJS/commit/f8a5fa33644e14b1b9f45f2fb767343208a9d111#diff-4d5aa81bf4f18104bb6ea53a8b5d1f43R15) property to false

To use travis-ci, you can login with github in https://travis-ci.org, and active travis to BackstopJS, then each push and pull request, Travis (currently), will install node modules and run `gulp lint`, if some error appear in this test travis will warn you about the error in the pull request. ([Here is an example](https://travis-ci.org/felquis/BackstopJS/builds/43340436))

Do you like it?