added support for varnish-devicedetect

When a site is sitting behind varnish the first device to hit the page sets the uagent_info for that whole page regardless of if a different device hits the same url directly after. The page is cached in varnish (php does not process), so the theme won't change for the next device.  

This change modifies the plugin to rely on $_SERVER['HTTP_X_UA_DEVICE'] and the setup depicted here if it is available: https://github.com/varnish/varnish-devicedetect
This moves device detection into the caching layer so different versions of the page can be cached per device.  If HTTP_X_UA_DEVICE is not available the plugin detects normally.

There is also a patch against -r696222 of http://plugins.svn.wordpress.org/device-theme-switcher/trunk/ here: https://gist.github.com/broderboy/5367090/raw/7a08e41bbef94d68fba25d240cd5a958fc79aa33/gistfile1.diff