Add delete_folder() method.

- Didn't add test case (sorry)
Fix delete_folder(), add tests for API conformity.

* delete_folder() added URL paramters of the form 'recursive=True',
  which is not supported by the Box api. Now they are of the form
  'recursive=true'.

* Test with recursive parameter and etag header.
Merge pull request #6 from echelon/master

Add delete_folder() method.