Better mock for file I/O

 The MockOpen class should work as a stand-in replacement for mock.mock_open
 with some added features (though it tries to conform to how the builtin 'open'
 works where the two differ).
 .
 Features include:
 .
  * Multiple file support, including a mapping-like access to file mocks by
    path.
  * Persistent file contents between calls to `open`.
  * All the regular file operations: 'read', 'readline', 'readlines', 'write',
    'writelines', 'seek', 'tell'.