avoid the 1-level-deep XPath selector hack

Immediately scope to root element if given an XML document node
update `get/post/put` signatures to bring more in line with Faraday

Also ensures that `post` & `put` have the same signatures, to avoid
confusion
don't have Collection enforce a cache key

That doesn't belong to its responsibilities
remove `objectid`

it's too application-specific
configure the Nibbler base parser class