Fixing Safari Downloads

Added a content-type header to the download action. Tried this out locally and
it stopped adding .html to the file and even extracted the zip into a folder.

This closes #55.

![screen shot 2014-09-05 at 12 53 59](https://cloud.githubusercontent.com/assets/94331/4164691/008d197c-34f9-11e4-9cdb-6dc5c5c17052.png)

Top example is the final result, and the previous ones are the broken ones.