PROPFIND variable depth and infinity depth support

This patch adds support for variable depth in PROPFIND requests. The client can set the Depth HTTP header to any integer value or to 'infinity'.

The server default is to support at best 1-depth request; every request with depth different from 0 or 1, will be forced to the server default.

A custom maximum depth can be enabled setting the environment variable WEBDAV_PROPFIND_MAX_DEPTH in the web server configuration.

A unit test has been added to test the feature.

See also http://code.google.com/p/sabredav/issues/detail?id=204