YAML support

Pretty much what the title says.
I find YAML format to be way more readable.

Based my changes on initial pull request by "cjroth".
+ Fixed failing test (cjroth changed API to use "json" instedad of ".json"
+ Added test for passing .yml to config.
+ Minor readme / message changes.
+ Found bug that if locale.js exists it overrides any locale.json and locale.yml

I see that you are running tests against many node.js versions on Travis ci, inluding 0.1.
It can cause issues since I used "fs.existsSync", which, I believe is a part of new API.
Anyway... CI will show. If bad things happen I'll revert to using old fs.statSync.