Added support for arbitrary tag directories.

Only works for directories one level deep. This seems to be what the spec calls for.
include tag directories in tagmanifest files
Ignored eclipse generated project file.
Added tag specs for bag.remove_tag, bag.delete_tag, and bag.add_tag.
Added tag specs for the tagmanifest files.

Fixed the description tag.
rewrite tag_files to list only tag files in the manifest
make tagmanifest_files compatible with new tag_files
implement persistence for bags; require manual add_bag_file for non-manifest tag files
Implement validation for files contained in a tag manifest.
improvements to bag-info.txt.
adds Bagging-Date and accepts other tags at time of creation.
add tests for Oxum method
initial payload oxum implementation
use hash to control default bag-info headers
auto generate payload oxum and include in bag-info.txt
write_bag_info ought to work with no params; give it an empty hash by default
test: add_file should update payload-oxum
update oxum when file is added
use update_bag_info for bag-info.txt persistence
Added tag spec to not force removal of previous values.
Merge branch 'size-oxum' of https://github.com/no-reply/bagit.git into size-oxum