Refactored default separator into constant, as there are multiple uses.

Also added .idea to .gitignore
Made 'tags=' method be able to handle either String or Array, and made 'tags_array=' alias to 'tags=' for backward compatibility.
Added spec for tags_array alias method
Merge pull request #2 from johnnyshields/master

Made tags setter be able to handle both String and Array + Mongoid 3 changes