Downloading video streams

Hello!
It would be nice to be able to download the videos. I use this feature for slow DSL lines in order to avoid waiting for the buffering. Moreover, it is a nice feature for backing up videos removed from the mediathek too early.

Thus, I implemented a download feature comparable to the one in the YouTube plugin. The dependency to the SimpleDownloader plugin (which is doing the work) is optional. The code informs the user if it is not installed. The changes have been tested in XBMC 12.2 (German) on Windows and Linux. More details are given in cfc650a433a87cb05fc844c9695277c08103a6b4.

Since the new feature seems to be useful for others as well, I send you this pull request. Just send me a short message, if further changes are required. Best regards, Florian