removed avconv: using melt for video encoding and generation

NOTES:
originally $(AVCONV) was "avconv" or "ffmpeg", but the argument -loop
was unknown to ffmpeg, and avconv was not in the project requirements
(to get avconv you have to install libav-tools).
Adapted travis libx264 to ubuntu 12 package
More adjustments for Travis...
Provided a MELT variable.
Merge pull request #103 from inaes-tic/darthpolly/makefile_fix2

removed avconv: using melt for video encoding and generation