Guessit

This branch use [guessit as filename parser](https://github.com/wackou/guessit). As this library is specialised in parsing, it should be more robust and it's easier to maintain.

All test cases are passing, so it should work in a real world configuration, but i need some alpha users to test this branch before merging this in the trunk.

What remains to be done before merging:

- [ ] A ```parser_type``` plugin to switch from guessit parser to previous internal parser, or any other custom one. Currently it requires [code modification here](https://github.com/Flexget/Flexget/blob/guessit/flexget/utils/parsers/parser_common.py#L18-19)
- [ ] At least 3 users have tested this branch on their configuration.