Defer session instantiation to first set

[pushrax]

@twistor brought this up in #323 and I agree – sessions shouldn't be created until necessary.

[brendensoares]

https://github.com/revel/revel/blob/master/session.go#L155
https://github.com/revel/revel/blob/master/session.go#L147

So, how about this:

if len(c.Session) > 0 {
    c.SetCookie(c.Session.cookie())
}