Small improvements to runInstaller

- Removed `cb` parameter as it's not used & misleading.
- `options` is now optional
- Improved docs for `runInstaller` a bit. Still needs more work though.
Merge branch 'master' of https://github.com/edjafarov/node-webkit-updater into runinstaller-cb

Conflicts:
	app/updater.js
Merge pull request #32 from adam-lynch/runinstaller-cb

Small improvements to runInstaller