Add support for Python 2.X

There are no `PyUnicode_AsUTF8` and `PyUnicode_FromStringAndSize` in Python 2.x. Instead, we should use `PyString_AsString` and `PyString_FromStringAndSize`.

The patch has been tested by comparing results of `houdini.escape_html('<p>哈</p>')` in Python 2.7 and Python 3.3.