Adding a remove relation button to the service relation viewlet when the relation is in error.
added test to make sure that the remove relation button shows up
Merge pull request #73 from hatched/remove-rel-button

To QA:
Deploy two relatable charms and relate them.
Scale one to 100 units.
View the relations tab and wait for one to go into error.

Once you see a relation in error hit Ctrl+s to disable the simulator.
Click the 'remove relation' button
It should pop up a dialogue, confirm the dialogue
The relation should now be gone.