LoginPage request handler updated

LoginPage request handler will no longer create a duplicate user session if a user is already logged in. Half fix #22.
JsonLogin now responds to logged in user

If a user is already logged in an HTTP/1.1 400 Bad Request will be sent as a response.

Updated the docs to reflect this change.

Closes #22
Merge pull request #28 from roberthodgen/fix-22

Login mechanisms check for logged in user