Rely on just one XML library

[exarkun]

Currently txAWS uses both lxml and the stdlib xml libraries. This is confusing and results in duplication of efforts. Pick one and just use it everywhere.

[mithrandi]

Historically, lxml on PyPy has posed some difficulties, but as things currently stand, lxml works just fine on PyPy, so I suggest we pick that. (Among other things, lxml's XPath support is quite useful in many scenarios, although I don't think it's being used anywhere yet)

Mixing uses of the stdlib ElementTree and lxml.etree is probably going to result in somebody trying to pass objects from the one library into the other library at some point, which is going to result in very confusing failures.