fixed possible index out of range error
since not all streams are broadcast in multiple resolutions, it could
have happend that an items was not playable because no proper stream
was found.
Consider the following Scenario: User wants maximum 720p, stream is
only available in 1080p. Hence, the stream is removed from the list.
By accessing streams[0], the index is out of range and no stream will
be available.

Now we take the smallest possible resolution and improve our guess by
searching for a better resolution smaller or equal to the chosen
maximum resolution. That way, we always return a stream.
Merge pull request #22 from ccaspers/master

Bugfix for TwitchResolver