Adds hability to copy magnet links to the clipboard

This adds a new keybind, M, to copy magnet links to the clipboard.

I implemented this for personal use and I am not entirely convinced that it belongs "upstream", especially because of the second point. But decided to submit it anyway.

* Magnet links were incorporated in Transmission 1.8 (RPC 7). Since TRNSM_VERSION_MIN and RPC_VERSION_MIN are set to '1.90' and 8 respectively then I thought that this would not be a problem. If it is, then it would be pretty easy to disable this feature on older transmission versions and not cause any problem for then.

* I used an external library, [xerox](https://pypi.python.org/pypi/xerox), to handle the system clipboard. I did this because I did not find any native and portable way to archive this (this library claims to works on linux, mac and windows, I only tested it on linux). If anyone knows a more "popular" library to do this, then this could easily be changed. If there are multiple alternatives we could even use some kind of fallback mechanism (like the way is done with simplejson).