Added support for adjustable linear color stretches.

- spectral.py: Added imshow_stretch and imshow_stretch_all to settings.

- graphics/graphics.py:
  - (get_rgb): Change `stretch` keyword to define linear contrast stretch.
  - (get_rgb_meta): New function - same as get_rgb but also returns metadata.

- graphics/spypylab.py:
  - (ImageView.set_rgb_options): New method to change RGB display (adjust colors
      stretch, color table, color scale, etc.)
  - (ImageView.open_zoom): Use self.rgb_kwargs (new member)
  - (ImageView.__str__): New member to return basic view parameters.

- algorithms/spymath.py: New function get_histogram_cdf_points.n

Change default number of color scale color levels.

- graphics/colorscale.py:
  - (ColorScale.__init__):
    - Default number of tics to number of colors.
    - Use np.linspace to set tic points.

Changes for python3 support

- io/aviris.py: Check for python3 in read_aviris_bands.