Support for Groovy 2.0.1, asm 4.0 and gradle 1.1

Here is pull request based on spock-0.6-groovy-2.0 tag and groovy-1.8 branch that supports the new asm 4.0 part of the new Groovy 2.0.1 version.
The build pass and we are starting to use it on our Groovy 2.0 project.

I think it should be a new git branch groovy-2.0. 

Hope this helps,
Fred.