Added HTTP-based authentication

The purpose of this is in order to tie CalDAV accounts to an external web
application. This application must provide a URL end point to which a POST
request can be made, with the username/password sent as the payload. A 200 or
201 response is considered successful authentication. Any other response is
a failure.
Merge pull request #15 from MelianLabs/http

Added HTTP-based authentication
Clean the http-based auth module