Other implementation of multiple IE files

My take at #846, conflicted with #867 

I only ported Youtube and Generic for now, but it's working.

* I define `InfoExtractor` and `SearchInfoExtractor` in `youtube_dl/InfoExtractors/InfoExtractor.py`
* Each website gets its file, maybe with multiple IEs (this makes it possible to share functions, for example)
* IEs are listes in `youtube_dl/InfoExtractors/__init__.py` as a list of `(SITE_NAME, [IE_NAMES, ...])` tuples
* `Makefile` and `setup.py` are updated to handle `youtube_dl/InfoExtractors/*.py` and to build correct zip and exe
* Tests are updated to use `get_info_extractor`