Refactor move / rimraf to ES6

[JPeer264]

(#355)
move:

I renamed the function from mv to move, it shouldn't be a breaking change

rimraf:

Here I put module.exports to the end of the file, as it is the same in all other files. Also I refactored one line if-statements, also with the same reason that it is the same in all other files.

---

I skipped copy/ncp as discussed in #360 (comment). Now I will start with the tests, unless I missed anything. I will make a pull request for each folder (copy, copy-sync, ...) if that is ok for you.

[coveralls]

Coverage increased (+0.3%) to 85.434% when pulling b9d8b45 on JPeer264:feature/refactor-es6 into f831b05 on jprichardson:master.

[jprichardson]

Thanks @JPeer264!