Add support for Mix-IT 2013 conference

This pull request contains the following features:

- move crawl and other functions into a devoxx object in order to provide support for multiple conference API;
- create a mixit object with its own crawl function.

the `events` array contains the list of conferences to be crawled.

Some "limitations" of the Mix-IT API:
- all types are of type "Talk"
- the presentations have to "track" property
- a "virtual" room `???`is created for talks with no defined room property