Performance and Player class changes

Stats are no longer CommaSeperatedListFields, each stat has its own
attribute. Performance objects can be created w/out kargs (useful since
there's 21 attributes now). Players objects have a method for finding
the sum of a stat across performances.
Merge branch 'new_model' of https://github.com/chrisbacon/rugby_tracker into new_model

Conflicts:
	rugby/models.py
	rugby/tests.py