Implements drag and drop upload of image in every markdown-area

creating issues, notes, wikis, and merge requests.

Samples:

Issues:
![issue_new](https://cloud.githubusercontent.com/assets/4653624/3064737/436411ae-e258-11e3-8cef-06760490d91b.gif)

![issue_new_result](https://cloud.githubusercontent.com/assets/4653624/3064738/50f08064-e258-11e3-9dd2-293f846233b3.gif)

Notes:
![notes_new](https://cloud.githubusercontent.com/assets/4653624/3064747/75906e2a-e258-11e3-85e3-9bd1710d7f8e.gif)

Wiki:
![wiki_new](https://cloud.githubusercontent.com/assets/4653624/3064762/b953a9a6-e258-11e3-8d8c-b1d2e2c75ba3.gif)

![wiki_new_result](https://cloud.githubusercontent.com/assets/4653624/3064766/c3612400-e258-11e3-9e3b-a11984a77e9e.gif)

Merge Request:
![merge_request_success_out](https://cloud.githubusercontent.com/assets/4653624/3064776/e2b13084-e258-11e3-9626-01bf70a46f82.gif)

![merge_request_result](https://cloud.githubusercontent.com/assets/4653624/3064787/14dca1e2-e259-11e3-92c3-519f4070a480.jpg)

![merge_request_error](https://cloud.githubusercontent.com/assets/4653624/3064793/1e62a676-e259-11e3-99ba-7d2e907cc44f.gif)

Right now, we're writing the spec for this. Again, feedbacks are needed.

Note:
This is a follow-up PR to #6962. We just entertained the idea to generalize this feature to have a more unified UI/UX.