Pass grailsHome to launcher via system property when available

Grails depends on grails.home being set for some features.  This plugin already
had some support for providing a grailsHome via project property (which can
either be set on the command-line or by assigning a value to "ext.grailsHome" in
the build script), but this value wasn't being passed to the launcher so that it can
be used.  In particular, after this change, if you set the grailsHome project property,
when running the grails-run-app task, Environment.isDevelopmentMode() should
now return true.

I attempted to create an integration test for this, but didn't come up with a good way to get it working repeatably across different environments.  In order to test it, the test would need acces to a grails installation.