Fix broken import in Python 3.x

In Python 3.x, the import statement works slightly differently, particularly with relative imports. The statement as it was worked properly in Python 2.x, but raised an ImportError in 3.x. By making the import explicitly relative, it works in 2.x and 3.x.

For additional information, see: http://stackoverflow.com/questions/12172791/changes-in-import-statement-python3

This change should have no impact whatsoever in how the library functions.