Merge pull request #5 from PutmanMedia/pulls/limit-fixes

Respect limit_to_holder in existing categories list, more efficient quer...
Merge pull request #2 from PutmanMedia/pulls/urlsegment-saved

BlogCategory-> LookForExistingURLSegment() fixes
Update README.md

Added a note about adding the "BlogCategory" tab to the BlogHolder class.
NEW Add categories on entry, default to current holder

Its a bit confusing to have CMS authors go to a different page
(BlogHolder or BlogTree) in order to add categories, particularly
if they can edit the existing ones directly on the entry already.