Updated NZBs'R'US provider and added backlog searching.

I used the other providers for an example and have overridden *_get_title_and_url_* due to differences in the XML returned between the RSS feed and the API, if anything needs changing let me know as I am not that familiar with Python.

Changes:
* Updated RSS feed to include the SD-x264 and misc groups.
* Updated labels for credentials to reflect naming on NZBs'R'US website.
* Added backlog searching through the API documented [here.](http://www.nzbsrus.com/NzbsrusAPI.pdf)