+ added whoosh to package requirements
+ added __indexed__ attribute to Model
+ added docstring content for __required__ and __indexed__
* refactored TypeEnforced.__setattr__
+ added get_next method
* changed all instances of 'id' to 'pk' to avoid name clobbering.
* moved pk in Database.update signature, changed to keyword arg
* changed all instances of "id" to "pk" to avoid global name clobbering
+ added update code to Model
+ added better type enforcement