simplify code, extract common part of a dial functoin
Added a way to change configuration of session storage MaxAge

Current implementation of Codecs don't expose that a default codec used by
redistore is securecookie.SecureCookie which is aware about *max age*.
The problem is that MaxAge from `SecureCookie` is nowhere connected to the one
form `RediStore`. If we change RediStore.Options.MaxAge it doesn't affect
SecureCookie Codec configuration. More details about useage in
`RediStore.SetMaxAge` function documentation.
Merge pull request #12 from AgFlow/master

Added a way to configure storage MaxAge.