Modify xhtml_unescape to handle hex numeric refs

Modify xhtml_unescape to handle hex numeric refs

While the existing code handles references like   properly,
it doesn't handle the valid case where the number is specified
in hex. For example 'foo bar', 'foo bar' and 'foo bar'
should all decode to 'foo bar'. The changes in escape.py check
for the 'x' and decode appropriately. Also added unit tests for
unescaping strings with numeric references.