Notes for added/removed labels to/from an issue.

The idea of this feature is to create a note each time a label is added/removed to/from an issue.

Implements feature request:  http://feedback.gitlab.com/forums/176466-general/suggestions/6623258-create-a-note-when-issue-labels-change

Screenshots:
![screenshot from 2014-11-04 11 00 48](https://cloud.githubusercontent.com/assets/4246830/4896962/65bc3314-6401-11e4-97a7-60f6c97e2ce7.png)
![screenshot from 2014-11-04 11 01 12](https://cloud.githubusercontent.com/assets/4246830/4896964/672d84be-6401-11e4-9e2f-1b53800b7ad9.png)
 


This should just be `old_labels = issue.labels`.