Added MaxLength support to RediStore

Added MaxLength support to RediStore to allow for larger session values. Tests included. Triggered from https://github.com/boj/redistore/issues/4 and https://github.com/gorilla/sessions/pull/17

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.