Support paste file uploads

Summary:
This is so freaking cool that I will try to implement it also on Facebook.
Idea is from
http://strd6.com/2011/09/html5-javascript-pasting-image-data-in-chrome/.
I don't know how to properly detect support but lying about it is not a big
deal.

Test Plan:
Go to revision comment textarea.
Paste some text data - works as usual.
Paste some image data in Chrome - file is uploaded and a link to it is inserted.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1681