Video streams

These changes allow setting a time for Video Assets. This is useful when switching to live streams which are endless. For example you could change to a surveillance cam for 5 min and then come back to the next asset. Or you can switch to a live TV channel streamed by some server.

The default behaviour is unchanged, when the duration is set to -1. In this case Screenly tries to figure out the length of the asset and put it into the asset DB just as it does now.

What this pull request also does, is accepting rtsp://  URIs, as this is the protocol used by many IP cams.