fix get_thumbnail to download entire graphic

[VoiceOfSoftware]

This is my first time using GitHub, and first time writing Python, so apologies if I'm doing this wrong!

My issue was that downloading thumbnails did not retrieve all of the data from The Frame. A 59K png only received about 3K, and then stopped. So I added a while loop that keeps downloading until the socket is finished sending data (e.g. header['fileLength'])

This was my test code that was failing to download the entire png:
thumbnail = tv.art().get_thumbnail('MY_F0011')
file = open('MY_F0011.png', 'wb')
data = file.write(thumbnail)
file.close()

[xchwarze]

thanks for pr!