Few improvements

Hi there,
first of all, thanks for this library, you're saving me a lot of time. I run a subtitles website and i have to manipulate srt files very often.

i made some changes you might want to merge in your master branch:
- I splitted the two classes in two different files for more readability
- I added namespaces to the classes
- I added a composer.json file in order to easily install your library through the composer


let me know!
thanks