Changed serialization method, added version with Redis SELECT DB.

* Use gob instead of securecookie for session value serialization.
Instead of relying on securecookie for both cookie encoding and maxLenght
validation for both `session.ID` encoding and `session.Value` serialization
I implemented a simpler version, where `session.Value` is not tight to
securecookie Options and encoding. Storage has his own MaxLength parameter
And use it to validate serialized `Value` length.
* IsNew is also true when no data available on Redis
* New `NewRediStoreWithDB` function with DB selection parameter