Add skimage.viewer.qt wrapper.

This allows the viewer to use either PyQt4 or PySide as the toolkit.
Add PySide compatibility for save dialogs
Fix file open dialog for PySide
Remove `alignment` kwarg for compatibility with older PyQt4
Remove `alignment` kwarg (left out from previous commit).
Merge pull request #551 from tonysyu/pyside

Add qt interface for PySide & PyQt4 compatibility in skimage-viewer