Reorganize parsing logic, attempting to simplify traversal.

 - handle_microformats: calls parse_props recursively on its *children*
   rather than on itself with a special flag for is_root_element

 - parse_props: only recurses in to child properties if the current
   element is not part of a nested microformat (otherwise, the nested properties
   will be handled by the recursive call to handle_microformats)

 - added a test for the case Kartik discovered, where p-names in nested
   h-cards were hoisted to the parent h-card

 - use assert_equal instead of assert ... == in nose tests for more
   informative print outs

ref issue #29