Second beta for v2.0.0

- fixed bugs in scaling dash pattern by linewidth
- fixed over/under propagating in images
- back ported improved png API from master
- deprecate gtk, gdk, wx (not gtkagg, wxagg) backends
- tweaks to default font sizes
- ensure default ticker always has at least 2 ticks
- remove vendored version of `six`
- updated GUI icons
- many small bug fixes