Remove dependency on simplejson

The simplejson module is included as json in the Python
standard library as of 2.6, so no need to depend on it.
However, this change will use simplejson over standard library
json if it exists.
Stop FutureWarnings from ElementTree.

"The behavior of this method will change in future versions.
Use specific 'len(elem)' or 'elem is not None' test instead."