Misc fixes and updates

- enhanced MultiDim plugin: now has more information in widget for
  choosing HDU, animation feature (a la ds9) for data cubes, and
  support for showing spectral coordinate
- updates to documentation
- fixed a bug in PixTable plugin where table would disappear if the
  plugin was closed and opened again
- improved color of readout line on gtk
- changes to Bindings to allow new modifiers to be defined and more
    flexibly used
Updated bindings examples, bumped release