Add test about format guessing

The test does not pass.

See issie #19.
Revert "Make the guess_format finction independant of the iterator nature"

This reverts commit 3bc7c8ba7fa491aef0bd9133e2fa21733f0703aa.

This change seems to break format guessing.
Add cover to the gitignore

By default, nose create html coverage report in the cover directory
instead of htmlcov. Add cover to the gitignore.