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'

bin/release-_pre.sh

@@ -6,4 +6,6 @@ npm run update-authors
 git diff --exit-code
 rm -rf node_modules/
 npm install
+npm prune
+npm dedupe
 npm test