Time shifting methods

Hi christopher,

Thanks for the solid srt parser implementation!
I added two methods for timeshifting (non-progressive and progressive) plus some changes to the internal time calculations to use regular floats instead of DateTime, resulting in considerably faster execution (2+ times faster) and allowing a sleak implementation of time shifting compared to DateTime. I did write also write tests and a quick summary in the README.md file. The tests pass and I also successfully used it on a real life problem i was having. (Spanish subtitles for Blade Runner :))

If you like the changes, i'd be happy to see you integrate them! :)

Best,
Simon