Set up tests and add them for Path.

As per suggestion from you-know-who, here's to a good start of unit tests for Keystone.js! ;-)

- Added tests for the `Path` class. It was independent and easy to get going.  
  That's also a useful way to get into unit testing — pick some low hanging fruit. And in no time you'll be at >90% coverage. ;-)
- Threw in a Makefile for quick access and keeping it simple. Just invoke `make test`.  
- Renamed `/tests` to `/test` as a couple of those testing tools default to it.  
  If you're keen on having the folder name in plural, I'm sure there's a way somewhere.
- Used tabs as per `.editorconfig`.
- Missed semicolons in tests. :-/  
  If only JS had a way to add them automatically... something like an automatic semicolon insertion mechanism. ^_^ But seriously, if you find those are necessary, I can cobble up a sed script and sprinkle some. :)

Let me know if you've got any questions.

PS. And yes, Must.js comes with support! Both on-site and over voice calls.