Make dropzone message clickable

In the current build of Dropzone, you can click anywhere in the drop zone to bring up the browse dialog *except* the "Drop files here or click" message, which is a little odd. This patch should fix that. 

I wasn't able to test the patch as-is, because I couldn't build properly with grunt (stylus complained that it couldn't find nib, and the built JS file didn't seem to work). I did test basically the same patch in (a slightly older version of) the checked-in JS file and basic.css, and it worked. So, hopefully it will work for you :)