Add top-level release target - initial version

Adds a 'release' target to the top-level build.xml that creates a tar bundle (in tmp) containing the toolkit and the sample toolkits.

The tar automatically includes the toolkit version and date of the build, note this does not yet include a build identifier in the version number. This is just groundwork for that step.

The bundle is intended for SPL developers that want to use the toolkit, not developers who want to modify the toolkit.

Pull request intended as an initial version, to get feedback on the contents etc.