Adding Spock integration test infrastructure

I've added the option of integration testing resources using Spock as well as JUnit.
This is the summary of my changes:
<ul>
<li>Added a dependency on the Grails Spock plugin.</li>
<li>Specified the common itest helper methods in an interface and placed the default implementation in a mixin class so that it can be shared.</li>
<li>Added a base integration specification.</li>
<li>Cloned and ported existing test cases to Spock specifications.</li>
</ul>

I know this is a hefty modification, but I'd really like to see this functionality included; so don't hesitate to post any comments/concerns.