Fix imports for the new version of trollius, which changed its package name.
Small security improvements to xsrf tokens; add tests.

Use os.urandom(16) instead of uuid.uuid4(), to reclaim a few bits of
entropy.  Use _time_independent_equals for comparison.
Change the xsrf cookie format to be masked with a random salt.

This protects against the BREACH attack.
Add the option to request an older xsrf cookie version.

Fix an issue with v1 cookies on py32.
Docs for xsrf cookie change; 3.2.2 release notes.
Set version number to 3.2.2