Restructuring and EnumField

Hi, 

We're proposing here a heavy restructuring of the repository so that it's more convenient for installation through pip and ssh : ```pip install git+ssh://git@github.com/MongoEngine/extras-mongoengine.git#egg=extras_mongoengine```.

Aside of that we're also proposing a new ```EnumField``` field which implement a seamless integration of enumeration of pip package ```enum34```.