Install hosted apps from querystring Fixes #743

review? @mykmelez

Myk, this allows you to install a hosted app by appending the querystring ?install=[encoded manifest path] where [encoded manifest path] is a url to a hosted manifest that has been base64 encoded, then URI encoded.  For example:

`encodeURIComponent(btoa('http://www.mykzilla.org/app/manifest.webapp')); // -> "aHR0cDovL3d3dy5teWt6aWxsYS5vcmcvYXBwL21hbmlmZXN0LndlYmFwcA%3D%3D"`

Try appending `?install=aHR0cDovL3d3dy5teWt6aWxsYS5vcmcvYXBwL21hbmlmZXN0LndlYmFwcA%3D%3D` to the URL of the dashboard, once it's open.

The reason for the base 64 encoding is that only doing URI encoding doesn't work for resource:// URL's; FF reports the URL as broken and substitutes some of the characters.