Raise more specific exceptions for error conditions recognised by xml4h
Builder tests now pass for (c)ElementTree adapter
XPath (and all) tests now pass for (c)ElementTree adapter.

This is more of a capitulation than an accomplishment. I gave up trying
to make ElementTree XPath queries perform any better than the underlying
library supports, and rewrote the tests to avoid asking anything
difficult of this implementation.

Remember to note "limited" xpath support in documentation for users of
(c)ElementTree adapter, per http://effbot.org/zone/element-xpath.htm
Only ElementTree versions 1.3+ and 1.0.6+ (cElementTree) are supported
Fix transplant of ElementTree nodes to delete original nodes.

This required broad changes to make the transplanted node's original
parent available to the ElementTreeAdapter, since it's not always
possible to lookup the parent of a given node in ElementTree.