Always remove sources and set src of video tag in first-frame setups

Internet Explorer (9) always prefers source tags before src-attribute
it also prevents touching the sources tags while playing the video so
the sources have to be removed before playback to get playlist item
switching working.

Fixes #345
Clear rewind to first clip when playlist item explicitely chosen

Fixes #342
Merge pull request #355 from flowplayer/bug/342

Clear rewind to first clip when playlist item explicitely chosen