Changes due to photo/frontend tag bugfix

Now that photo/frontend#927 is resolved, the tag.create endpoint returns true when a tag is created successfuly.

This PR updates the python classes to match this behaviour, and ensures that tag names are correctly escaped when used in a URL path.

The tests have been updated to match, but there are a couple that fail:
 * tag.update can't be tested, since there are no tag attributes that can be updated
 * Tags with double-slashes can't be deleted (photo/frontend#919)

@walkah: let me know if this looks ok.