Added support for SRT subtitles (and on/off toggle option) & BIF trick files (FF/REW on Roku)

Added support for SRT subtitles (and on/off toggle option) & BIF trick files (FF/REW on Roku)
Updated rokuencode.sh script to output SRT subtitles & BIF trick files (FF/REW on Roku)
rokuencode.sh script won't die if bif/subtitles utils aren't available on the system