fix edit_relationship

Based on what was done for add_url and https://wiki.musicbrainz.org/User:Bitmap/Relationship_Editor_Parameters I've tried to fix the edit relationship method. Please realise I know very little of python and nothing of POSTing data so this needs a thorough read over! 
From what I understand of bitmap's instructions the method will now be required to submit the gids of the entities so I have added these - therefore calls will need to be changed elsewhere.
I also don't think we'll be able to check the old_link_type as before so I've removed that (correct?)
I haven't implemented multivalue attributes or the 'ended' boolean. 
I was particularly unsure about whether I'd done the dictionary bits right?
Thanks!