Added a typemap and wrappers for SBInputReader callbacks

Now it's possible to use SBInputReader callbacks in Python.

We leak the callback object, unfortunately. A __del__ method can be added
to SBInputReader, but we have no way to check the callback function that
is on the reader. So we can't call Py_DECREF on it when we have our
PythonCallback function. One way to do it is to assume that reified
SBInputReaders always have a Python callback (and always call Py_DECREF).
Another one is to add methods or properties to SBInputReader (or make the
m_callback_function property public).

llvm-svn: 162356