Use lxml for processing XML

The lxml XML toolkit is a binding for the C libraries libxml2 and libxslt.  It
is compatible with ElementTree API, which is more pythonic than the DOM
interface.  In addition, lxml has attractive features like XML validation and
namespace mapping.

Replacing the current DOM interface with the lxml API will simplify Swift3 code
a lot and improve maintainability, I believe.

Change-Id: Ie2291f180421559ed3320a173d2f7eea81f459d9
Merge "Use lxml for processing XML"