tag editing withing invoice editing

[jzarate]

check that feature

[jzarate]

r227 added tag adding/removing functionality.

NOTE TO OTHERS:

• this op is fully ajax, cause tagging isn't related with classical invoice ops, so it shouldn't need to be included in the general invoice form submit process. Other convenience reason is the fact that tags are a "behavior" of the objects, which lives "outside" them.

• when a user adds a tag, the tag is added at that time, there is no need of "saving" the invoice for that.
• when a user removes a tag, the tag is removed at that time, there is no need of "saving" the invoice.
• should somebody think otherwise, please let me know.

[jzarate]

r234 small correction. don't insert empty string as tag.

[jzarate]

r236 added previously forgotten tag partial