Windows Portable App (#417)

* packager: call callbacks consistently

Before this, the callbacks would not being called, which would lead to
an incomplete build on non-OS X platforms when trying to build all for
all platforms.

* packager: Always produce OS X update file regardless of --package option

This makes it consistent with how the windows build always produces the
.nupkg autoupdate files

* packager: fix duplicate npm install

Move "npm prune && npm dedupe" into the release script. Remove an extra
"npm install"

* Make Windows portable app

When a folder named "Portable Settings" exists in same folder as
WebTorrent.exe, then use it instead of the default application config
path.

Closes #358

* packager: remove redundant signing warning

* cross platform zip function

* Set config file path to match config.CONFIG_PATH

* portable app: make electron settings portable

* portable: fix poster/torrent paths

* use cross-zip

* portable app: default download folder inside 'Portable Settings'

renderer/state.js

@@ -1,4 +1,5 @@
 var electron = require('electron')
+var path = require('path')
 
 var remote = electron.remote
 
@@ -255,6 +256,8 @@ function getDefaultSavedState () {
         ]
       }
     ],
-    downloadPath: remote.app.getPath('downloads')
+    downloadPath: config.IS_PORTABLE
+      ? path.join(config.CONFIG_PATH, 'Downloads')
+      : remote.app.getPath('downloads')
   }
 }