* 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)
