Fixed: Bug in is_valid method.
New: Automatically add an empty .keep file to preserve empty directories.
Fixed: Updated bagit spec url.

A more stable url should be used for testing fetch, for example a specific version of jquery from google cdn, which is almost guaranteed to not change.
New: unit tests for testing is_valid method.
Merge pull request #1 from DeepanjanRoy/master

Merged. Thanks!