Role class which can be handled as a set
Consider role function for equality operator
Refactor filter functions inside Role class

There's no need for the closure in these functions.
Small reordering of methods
Add role set tests and reword docstring
Merge branch 'role-set-with-tests' into jokull

* role-set-with-tests:
  Add role set tests and reword docstring
  Small reordering of methods
  Refactor filter functions inside Role class
  Consider role function for equality operator
  Role class which can be handled as a set

Conflicts:
	schematics/serialize.py
	tests/test_serialize.py