Tentative start of ElementTree implementation, tests run but fail
Progress in ElementTree adapter implementation, some tests now pass
Merge branch 'develop' into feature/ElementTreeAdapter
Merge branch 'develop' into feature/ElementTreeAdapter
All node tests except xpath queries now pass for ElementTree
Parser tests now passing for (c)ElementTree adapter
Tests in test_writer now passing for (c)ElementTree
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.
Merge branch 'develop' into feature/ElementTreeAdapter
ElementTree adapter code cleanup
Avoid creating adapter class instances when not necessary.

This change should improve performance, but most important will make it
possible to do  per-document caching of information in adapter instances.
Make it possible to manually clear cached data in adapters

This is only useful/necessary for the ElementTree adapter so far.
Cache ancestry lookup data in ElementTree adapter for performance
Update ancestry info in (c)ElementTree adapter when child node added, avoids unnecessary dict regeneration
Make separate adapters for pure-Python and C-based ElementTree library.

Make it possible to create an xml4h adapter for either the C or non-C
ElementTree implementation, rather than assuming that the C version
should be used automatically and always when it is available.

This will allow users with existing non-C-based ElementTree-based code,
or those who prefer not to use the C-based version for some reason,
to take advantage of xml4h without weird incompatibility errors.
Code cleanup of ElementTree adapter implementations.

Removed unused/redundant code, fixed incorrectly-scoped ancestry caching
mechanism, improved test coverage