Add upload button with support for .tar files

Few things worth noting

* It only supports individual files or .tar files at the moment, but it could easily be extended to all kinds of archives
* the javascript `uploadFile` function doesn't call `saveCurrent` like `newDoc` and the rest, because I couldn't figure out how to get $(...).click() working inside of promises
* I'm using the `system` function to call `tar`, and it takes in arguments from the browser. This should probably be done better, but I'm not sure it even matters since users own their sandbox environment already...
* You should check to make sure sandstorm-files.list is correct. My system has ruby 2.1 installed, and it was causing all sorts of extra crud to show up in it. I removed it as best I could, but I'm not sure I got all the dependencies.