Add xsrf.py from head of webapp2_extras
Add unedited xsrf tests from webapp2
Change xsrf test to run in furious
Move xsrf test to tests module

Add __init__.py to extras, hadn't done so in this branch.
Move and rename xsrf test
Merge pull request #62 from johnlockwood-wf/add_xsrf

Add xsrf.py from head of webapp2_extras