Fix bug in create_if_needed

Closes #12.

The logic is as follows:

* If the FS is "none", there is NOTHING to format. Period.
* If the FS is something else, we format it IFF it the block device
  doesn't have it yet.
Update the README.
Merge pull request #14 from piojo/readme

Update the README.
Merge pull request #13 from piojo/bugfix-stijn

Fix bug in create_if_needed