Fix PyMimeData coerce method to inspect lists for PyMimeData instances, tracking pickle status.

Also on failure of pickling, try to return a non-picklable PyMimeData.
Warn if can't pickle; handle PyMimeData subclass coercion better.
Start of some tests for qt4 clipboard.
Improved format detection.
Lots more tests (and they work!)
Merge pull request #127 from enthought/fix/qt4-drag-coercion

Fixes for PyMimeData coercion