MessageVerifier

Pulled this out of Dynamo and made a few tweaks to bring it up to speed
with the latest elixir version.
Change example session key name to something more clear.
using `Plug.Test` already imports `Plug.Conn`
Doc fixes
Session Cookie Store
Add a lower level test to prove that cookies are encoded
Start error messages with lower case.
Fix specs to work with proper API
Clean up `get_session/2` to one line
Merge pull request #48 from scrogson/cookie_store

Session cookie store with message verification.