Fix memory leak in build_kv_error_map_info when building KV exception

When building the KV error map, build_kv_error_map_info(), creates two memory leaks by adding strings directly to the err_info dict using the PyDict_SetItemString().

To fix the leak:

1. created a PyObject* that stores the string
2. add the created PyObject* to the err_info dict
3. decrement the created PyObject* after it has been added to the dict

Leak:

    if (-1 == PyDict_SetItemString(err_info, "name", PyUnicode_FromString(error_info.name.c_str()))) {

        PyErr_Print();

        PyErr_Clear();

    }

    if (-1 == PyDict_SetItemString(err_info, "description", PyUnicode_FromString(error_info.description.c_str()))) {

        PyErr_Print();

        PyErr_Clear();

    }

Also, call tp_free() in the exception_base destructor (see associated ticket PYCBC-1382).