Factor out ChangeRequest creation into methods.

This makes it easier to create more complex change batches that
encompass the deletion or update of multiple resource record sets.

Also, in update(), don't clear out @create_options until after the
change batch is actually called. Otherwise the object's state will be
mutated even if the batch raises for some reason.
Add some documentation.
Merge pull request #155 from ab/route53-changerequests

Factor out Route53 ChangeRequest creation into methods