Extract creation of bag-info.txt into helper function
Add update_payload to recalculate payload & manifest checksums
renamed update_payload to save, added test for updating bag-info.txt
simplified by removing duplicates parameter to load_tag_file, which effectively means duplicate fields are permitted in bagit versions before v0.97 which seems in keeping with the robustness principle.
added a save example
this superfluous warning is a bit noisy/misleading when calling the save method
make sure bag is still valid after updating bag-info.txt
Merge pull request #28 from LibraryOfCongress/save

Save