Many packager improvements; Windows signing! (#413)

* Many packager improvements; Windows signing!

* Windows signing works now! (Certs are on an external USB stick that
must be plugged into the build machine during build. We can't do the
same for OS X because certs need to exist in the login Keychain to be
found.)

Fixes #219

* Signing is now optional (so OS X and Windows contributors can run
`npm run package` without errors)

* zip, dmg, and deb arguments are now passed in as e.g. "--package=dmg"

* Print a huge warning when signing is disabled so we're less likely to
ship unsigned binaries to users.

* Make console.logs during packaging consistent and parallel
("creating..." followed by "created.")

* More aggressive signing warnings

* Warn when building OS X app on non-OS X platform (because signing
will never work on non-OS X platforms)
* Warn when building Windows app on non-Windows platform (because
signing doesn't work yet on non-Windows platforms)

README.md

@@ -50,12 +50,15 @@ $ npm run package
 To build for one platform:
 
 ```
-$ npm run package -- [platform] [package-type]
+$ npm run package -- [platform]
 ```
 
-Where `[platform]` is `darwin`, `linux`, or `win32`
+Where `[platform]` is `darwin`, `linux`, `win32`, or `all` (default).
 
-and `[package-type]` is `all` (default), `deb` or `zip` (`linux` platform only)
+The following optional arguments are available:
+
+- `--package=[type]` - Package only one output file. `type` is `deb`, `dmg`, `zip`, or `all` (default)
+- `--sign` - Sign the application (OS X, Windows)
 
 #### Windows build notes