[cdi-dev] Disposer methods for producer fields

Patch for https://issues.jboss.org/browse/CDI-145 at https://github.com/jboss/cdi/pull/45 This one is pretty straightforward, but let me know any comments. Usual rules apply, this will be up for review for 2 weeks from today (review extended if we make substantial changes), and then merged into master. Of course, we can still discuss and change it after, but obviously easier on everyone if you can get any comments in now!