Use internal open in chunks and text

Changes the ``chunks`` and ``text`` methods to use path's own ``open`` method. In this way, a subclass that overrides ``open`` does not need to modify the other methods to make them work.
Perhaps this logic should be added to self.open() such that self.open() supports 'encoding' and 'errors' parameters. On Python 3, that could be a simple passthrough, and a compatibility shim can be added for Python 2. Then, the diff for .text() would be about the same as the diff for chunks above.