add search_cpasbien.py for tracker cpasbien.pe

Hi,

I just did a search/discover plugin for tracker cpasbien.pe

tested, and validated, just grabbed 94 torrent with it.

Only thing is you can set multiple category in the config, but in reality you SHOULD only set 1 because the tracker doesn't support multiple category search, and I don't know how to limit this in the code.

So if a dev can modify my code to limit choice to only one category, it would be great.

Regards.

ticket #2801 on flexget trac
Maybe it would be nicer to use `posixpath.join` for these. i.e. `join(base_url, 'recherche', config['category'], query_url_fragment)`