Added calls for BTSearch API

Added support for BT Search to SYNO.DownloadStation.
The following methods were added.

**startSearchTask**: Starts a new search job and returns a taskid
**getSearchList**: Returns a searchresult, this can be an ongoing job or a completed job. An object is returned with the property 'finished' (bool) stating if the job has completer or not and the property 'list' (array) containing items as objects.
**getCategories**: Returns the categories the DownloadStation offers
**cleanSearch**: Stops and destroys a search job along with the results
**getModule**: Returns the installed modules (ie: torrentsites)


**Known limitations**
Search jobs are bound to the session and only one search job can be performed simultaniously.