Initial IDE Support

This adds a DslContext annotation for closure parameters. A global AST transformation is used to add a DelegatesTo annotation to each parameter annotated with DslContext.

I copied the source of DelegatesTo from the Groovy sources because Jenkins and thus the Job DSL plugin still use Groovy 1.8 which does not provide the DelegatesTo annotation.

For details about IDE support, see https://groups.google.com/forum/?fromgroups#!topic/job-dsl-plugin/2-iuHl4tidI.

The next steps will be providing useful GroovyDocs for the DSL methods and changing the top-level DSL methods as pointed out in the post on the mailing list.