Update static ffmpeg URL for tests
Implement set methods for ffmpeg & ffprobe bin paths
make forgetPaths() static and update all tests respectively
author tests for setting ffprobe & ffmpeg bin paths
fix bugs found from running tests for manual set of ffmpeg bin path
update README for manual set of bin path
refactor getFlvtoolPath into a static method and implement setFlvtoolPath
update README for new flvtool path set method
author tests for flvtool2 path discovery and setting