Bug fixes

This branch fixes some issues:
- Fix #34: Creating a new category does not automatically add it
- Fix #33: Post can loose a category if its not in the category list when categories are added.
- Fix #30: Blank categories are displayed in the category list but not on the edit screen.
- Fix #28: A category containing a comma shows up as two categories on the edit screen.
- Fix #26: Refreshing the categories list forgets categories that were just checked.
- Fix #24: Post editor shows html entities in categories.
- Fix #23: Categories list shows html entities and code for doublebyte characters