MaxAge = 0 does not agree wtih SETEX

A cookie MaxAge and the TTL for SETEX aren't necessarily mutal.  A MaxAge
of 0 tells the browser it is a session, but SETEX will fail.  Instead
detect if the MaxAge value is 0 and default to something reasonable for
Redis.  This is currently 20 minutes, and can be set through the
DefaultMaxAge member.
Merge branch 'master' of https://github.com/AgFlow/redistore into AgFlow-master

Conflicts:
	redistore.go
Enable injection of the underlying *redis.Pool
Return error on Close
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.