update to array data type for ds9 viewer
Add a new type of viewer based on a ginga/matplotlib combo

initial commit
Updates to ginga-based viewer option

- supports close() on viewer objects
- loads and shares preferences for bindings from users home directory
- allows user to enter imexam mode simply by pressing "i" in viewer window
  - press "i" again to temporarily exit imexam mode, "q" to permanently exit
- adds a graphical indicator that imexam mode is being used
Merge remote-tracking branch 'origin/ginga-viewer' into ginga-viewer