Added ActiveRecord 4.1 support

Just found your project last night. It's just what we were looking for! We're on the latest rails so things didn't quite work out the box. I pulled down the rails source code and compared 4.0 to 4.1 and there seemed to be quite a few differences in it. Here is a brief summary of the changes I made.

  * ActiveRecord::VERSION is no longer there. I couldn't tell immediately if it was moved somewhere else but I started using ActiveRecord.version.segments which isn't the sexiest of solutions.
  * Updated association embed class to match what I'm seeing in 4.1 including calling out Reflection.add_reflection
  * Created the EmbedsOneReflection class to match the other classes I see now in 4.1 (BelongsToReflection, HasManyReflection.) This is instead of changing AssociationReflection.
  * Moved create_reflection no the instant to create on the class

Let me know what you think.