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)

bin/release-_post.sh

@@ -2,7 +2,7 @@
 set -e
 
 git diff --exit-code
-npm run package
+npm run package -- --sign
 git push
 git push --tags
 npm publish