Added a MaxLength method to RediStore to allow for larger session
values. Tests included.

As per https://github.com/gorilla/sessions/issues/2 - this allows a
session to exceed the 4096-byte limit commonly enforced by cookie-based
stores. Redis has a much larger limit: typically 512MB -
http://redis.io/topics/data-types.
Merge pull request #5 from elithrar/master

Added MaxLength support to RediStore