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.
Handle use in a subdir. Patch from Clint Adams <clint@softwarefreedom.org> adapted from http://redmine.kozea.fr/issues/996
Fix issues with base_prefix option
Add proxy_base_prefix option to help run in a subdir behind a proxy

I have a setup where Radicale is running as the normal process, and
nginx is acting as proxy for Radicale for requests under /radicale/.
This means that all requests to nginx that go to /radicale/ are sent to
a radicale server that expects requests coming at /.

Obviously, all the href need to have the /radicale/ prefix to have
things work. So a new option proxy_base_prefix is added to allow having
such a setup.

All href that are sent in the replies will have the proxy_base_prefix
prepended.
Typo: missing comma

sync-collection and expand-property where returned concatenated in
PROPFIND answers.
Merge pull request #25 from jon-d/fix_sync-command_property

Typo: missing comma
Initial *untested* support for database storage
Clean database storage code
Support SSL for IMAP authentication

Based on Nikita Koshikov's commit:
https://github.com/interlegis/Radicale/commit/000fc2a
Merge pull request #15 from MelianLabs/http

Added HTTP-based authentication
Clean the http-based auth module
Update copyright years
Fix modification time for not-created-yet collections
Add SQL schema
Merge pull request #24 from vuntz/radicale-in-subdir

Allow running radicale in a subdir, as well as in a subdir behind a proxy
Merge *base_prefix config keys into base_prefix (fixes #996)
Useless typo
Fix some corner-cases for requests at root path
Don't try to login with IMAP and courier when user or password is empty
Fix types of IMAP config parameters