Spimagine

Minor fix due to switch to imageio

- removed unnecessary dependencies
- added slice position as keyframe-able parameter
- aded Overlay data class

- faster loading of numpy arrays
- nearest neighbour interpolation option in settings panel
- inverted colormap option in settings panel
- osx bundle now uses the python3 version
- bug fixes

version bump

version bump
- now compatible with both python 2.7 and 3.5+
- transition to PyQt5