Merge pull request #18 from webnard/vtt

VTT utilities added to convert SRT files to VTT and shift time codes
Merge pull request #19 from webnard/youtube-fix

YouTube videos gracefully handle thumbnail retrieval failure.
Updating vtt.py to work with Unicode.
fromSRT works in unicode and with better checks for time codes.
Test to ensure that SRT to VTT conversion is accurate.