Changes and enhancements to the Ginga toolkit

New backends
------------
* There is now an Agg rendering version of the FitsImage object
  - uses the python "aggdraw" module for drawing
    - get it here  --> https://github.com/ejeschke/aggdraw
  - this will make it easy to support all kinds of surfaces because the
      graphics drawing code does not have to be replicated for each
      toolkit
  - see example code in /scripts/example2_tk.py
  - currently not needed for Gtk, Qt versions of the object

* There is now a Tk rendering version of the FitsImage object
  (ginga.tkw.FitsImageTk & co.)
  - renders on a Tk canvas
  - see example code in /scripts/example{1,2}_tk.py
  - you will need the aggdraw module (see above) to use it

AutoCuts
--------
* the ginga.AutoCuts module has been refactored into individual classes
  for each algorithm
* The Preferences plugin for ginga now exposes all of the parameters
    used for each cut levels algorithm and will save them

Etc
---
* additions to the manual (still incomplete, but coming along)
* lots of docstrings for methods added (sphinx API doc coming)
* many colors added to the color drawing example programs
* WhatsNew.txt file added
Merge remote-tracking branch 'origin/master'