Adds new JavaScript function "js_download"

"js_download" is a JavaScript function used to download data (text or binary) from the web server.

You can download the test case here -- you should be able to see a hexdump that shows what was sent.:
https://gist.github.com/wchen-r7/e4755de1b29f0b22cc80

Tested on:
* - IE6 + Windows XP SP3
* - IE7 + Windows XP SP3
* - IE9 + Windows 7
* - Firefox + Windows 7
* - Chrome + Windows XP SP3
* - Chrome + OSX
* - Safari + OSX