* Improve types for xml.etree.ElementTree
Update signatures to reflect the following peculiarities of the
ElementTree library:
- The elementtree library accepts unicode or bytes for most xml values
in python2, and coerces everywhere -- but in python3, only str makes
sense.
- In python 2, the library produces str or unicode instances
unpredictably, depending on whether the xml is decodeable as ascii or
not. In python 3, it always produces str instances.
- The parser functions accept unicode or bytes in 2 and 3 -- again, will
coerce individual instances so heterogeneous lists are ok.
- In python 3, the tostring functions produce bytes or str, depending on
the value of the 'encoding' parameter.
* improve docs
* Improve ElementFactory type by specifying dict of 2nd arg
