run seedlink client in thread

Hi Sebastien!
I had a look at your tool yesterday. I like it. :)
However I noticed the GUI freezing, especially when all old packets are updated already and new ones only come in slowly. Actually, the GUI main thread gets blocked by the seedlink client, as it never exits the run() method and stays in it forever.
So, I moved the code around a bit and refactored the subclassed seedlink client to run in a thread (and only update the stream, leaving the plot updating to the GUI). It might not be very nice/thread-safe right now but it seems to work..

Things to check:
 - thread safe handling of the shared stream object necessary? (locking?)
 - some of the plot updating logic is commented, maybe that needs to be reintegrated..

This needs some work before it can be merged.
I'll give you and Lion push rights to my fork..