Merge pull request #6 from AgFlow/master

Session SET & EXPIRE are squashed in a single command.
fixes #7 - see long desc for why

It turns out that this was an issue with redis-server 2.4.14, which
didn't like how the separate SET and EX commands were coming through.
It seems 2.6.16 works fine on the other hand.

I've set the command to SETEX as per @jsimnz's suggestion, which works
fine on both server versions.

I came across this myself while setting up a debian wheezy server.
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.
Update NewRediStore doc
IsNew is also true when no data available on Redis