Support for specifing checksum algorithm, multiple checksums

I know the bagit spec allows for multiple manifest files with different checksum algorithms, but have been unable to find any tools that actually support generating them, and we have a use case for it with our digital archives at Emory.

I've added command-line options that will let users specify md5, sha1, or both, and adapted the existing manifest-generating code accordingly.  As far as I can tell, everything is working and the generated bags are valid.  I added a couple of unit tests as well.  Please let me know if any additional work is needed before you can include this.