Add note for assignee changes

Implements the feature as requested in http://feedback.gitlab.com/forums/176466-general/suggestions/5168130-record-assignee-changes-in-the-comment-timeline.

When an issue assignee is changed a note is generated. The note is italicized to match the status notes (reopened/closed).

![issue_assignee_notes](https://f.cloud.github.com/assets/2394772/1811289/426fe2c4-6e4e-11e3-9e92-bba6b4a72c15.png)