New features:
- Subtitles extraction: can select just one language
- Subtitles extraction: destination directory is, by default, the same as source directory

Fixes and improvements:
- Added unit testing framework and 2 tests
- Fixed a bug in Movie#valid?
Merge pull request #1 from alxbrun/extraction-with-options

New feature (more options for subtitles extract) + a few bug fixed