Implements clipboard feature for markdown areas.

This is our (w/ @nmcalabroso) attempt to extend the drag and drop feature in every markdown area (#7011). We have tested this feature in Mac OSX (Mavericks) and hoping to receive more feedbacks (with different OS) from you. Unfortunately, chrome is the only browser that supports the html5 clipboard.

Note:
1. This PR doesn't support multiple files in clipboard. In case this event happens, the uploader is just sending the one at the top of the clipboard's list.
2. Some images during our testing are uploaded incorrectly so we tried these same images in github to see if this is the expected behavior and as far as we know, it is. We're suspecting that the clipboard is OS dependent so further testing will be appreciated.

Samples:

Copy-pasting to upload image in creating issue:
![copy_paste_issue](https://cloud.githubusercontent.com/assets/4653624/3100537/5a6e109a-e61b-11e3-950e-0a6ef3337b05.gif)

Output:
![copy-paste-result](https://cloud.githubusercontent.com/assets/4653624/3100541/5a9a65b4-e61b-11e3-89ad-42968224d614.png)

Copy-pasting multiple files:
![copy_paste_multiple](https://cloud.githubusercontent.com/assets/4653624/3100538/5a6e86d8-e61b-11e3-8806-d396f91805d6.gif)

Output:
![copy-paste-merge-result](https://cloud.githubusercontent.com/assets/4653624/3100540/5a99a7aa-e61b-11e3-92a2-26c36532deb0.png)

Copy-pasting from web browser:
![copy_paste_browser](https://cloud.githubusercontent.com/assets/4653624/3100535/5a6ce67a-e61b-11e3-8024-e599decab414.gif)

Output:
![copy_paste_wiki](https://cloud.githubusercontent.com/assets/4653624/3100539/5a6f096e-e61b-11e3-8a36-9f6761a0a295.png)

Error handling:
![copy_paste_error](https://cloud.githubusercontent.com/assets/4653624/3100536/5a6e0258-e61b-11e3-9b1b-5e4cd1ff10f1.gif)