Add 'https' option to allow secure dev server

There are some cases where it is useful to serve assets via HTTPS.
Includes a self-signed certificate that is used when serving the requests.
Should not be used in production, but neither should webpack-dev-server.
Merge branch 'https' of https://github.com/diurnalist/webpack-dev-server

Conflicts:
	bin/webpack-dev-server.js