Use get() instead of attrib.get() (shorter)
Have parse module provide find() functions

This changes the parse module functions to better match the ElementTree
API and extends it with findint(), findtime() and findbin().

It also passes the namespaces to all calls that require it without
duplicating this throughout the normal code.
Support various ElementTree implementations

When using a recent enough lxml, even Python 2.6 should work now. The
most important requirement is that the findall() function supports the
namespaces argument.

This also now catches all exceptions when parsing the PSKC file fails
and wraps it in ParseError because various implementations raise
different exceptions, even between versions (Python 2.6's ElementTree
raises ExpatError, lxml raises XMLSyntaxError).