Improve docs for session store and only generate cookie if necessary
`put` has arity `3`
Merge pull request #47 from scrogson/doc_fix

`put` has arity `3`
Delete the cookie on drop session
Add a test no cookie is created if there wasn't one before
The store does not actually receive a nil value
Fix read_timeout default in docs
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.
Ensure plug parses content-type with CRLF as LWS