Refactor describes for upcoming tests
Singleton#lock now takes a callback and returns a promise
Use singletonFactory function in #exists tests
Just like the #lock tests
Singleton#release now takes a callback and returns a promise
Rename test/index.js to test/Singleton.js
Add tests for MongoDb persister
Update Usage now that Singleton returns promises
Create better library intro description
Increment version
Due to addition of enhancements