set Delay

Added 2 functions to set a delay (positive or negative) for subtitles.
new functions shift and sync

2 new functions : shift (set a delay) and sync (scale subtitles), based
on gnome-subtitles (https://git.gnome.org/browse/gnome-subtitles/).
fixed comment
fixed comment
moved round function

moved round function to setStart and setStop.
Merge pull request #5 from gordielachance/master

new functions shift and sync