Feature/threadsafe

Use `Thread.current` to keep Redis connection

Also
- Split frivol.rb into multiple files: each module in its own file
- Split tests in separate files by functionality
- Remove Shoulda and Mocha (`Thread.current` was making Mocha unhappy)

Update Travis-CI config and Rakefile and make sure [everything is green](https://travis-ci.org/marcheiligers/frivol)