Clean up XHR refs to avoid possible circular refs

Also adds destroy() methods for enyo.Async and enyo.Ajax to
clean up a XHR reference and a timer reference that could be
orphaned.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #512 from enyojs/fix-xhr-doc-leak

Clean up XHR refs to avoid possible circular refs

Reviewed-By: Cole Davis (cole.davis@lge.com)