Changes to user interface bindings management and example reorg

The user interface mapping just got a bit easier to use.  Ginga now
provides a way to do most UI remapping just by placing a simple config
file in your ~/.ginga directory.  An example for ds9 users is in the
new "examples" folder.

Many simple examples were moved out of "scripts" and stored under
subdirectories (by GUI toolkit) in "examples".