Adding two new settings to control cookie behavior:

- STORMPATH_COOKIE_DURATION (how long a cookie will last), and
- STORMPATH_COOKIE_DOMAIN (the domain cookies will work for, if multi-domain
  support is needed).
Updating changelog to include new settings.
Making Flask session expire after the cookie wears off.
Adding docs on setting the cookie / session duration.
Updating docs.
Version bump!
Version bump!