Updating to close issue #1 noted by johnscancella. This changes the l…

…ibrary to include the following:

1) Support for Updating the Manifest file
2) Support for duplicate metadata tags
3) Protecting user written tags
4) Improved validation to include validation beyond the payload-oxum
5) Added functions to allow for bag-header modifications
6) Updated bag loading support for < 0.96 and the package-info.txt file.
7) Additional checksum option (sha384)