3.0.x

This pull request includes the following changes.

- Upgrades Jetty Container version to 9.0.x
- Bumps Servlet API version to 3.0.x
- Provided a couple of configuration options to override keystore path and password (this is often needed in a real world Jetty development environment to prevent frequent certificate errors)
- Added a Readme file in preparation for the next pull request (3.1.x).

**Note**: Please publish the plugin to Grails Central as version `3.0.0` once this pull request has been reviewed and merged.