Update go.mod: removes gorilla/context (gorilla#196)

Don't use t.Run in tests, not supported in earlier Go versions

This package is meant to work on Go versions going back to Go 1.3,
which means tests can't use testing.T.Run which doesn't exists in Go
1.6 and earlier.

Adds support for SameSite cookie attribute (gorilla#165)

Add AUTHORS file; update LICENSE (gorilla#158)

[docs] Updated sessioncascade project URL. (gorilla#94)