Allow custom validation message to be passed in options object.
Added tests to check custom message behavior.
Return original error message.
Updated mongoose dependency to 3.8.x
Merge branch 'master' of https://github.com/pontifier/mongoose-unique-validator into pontifier-master

Conflicts:
	index.js
Refactored tests to remove duplicate methods.
Passing through message as an option.
Added documentation for custom error messages.
Incremented version number.