DOC: image tutorial touch-up

- expand some text regarding backend usage and IPython notebook plot
- improve style a bit (PEP 8)
- shorten array printout
- convert image resizing example to use PIL
- replace pyplot discussion in top material
- remove matplotlib argument to IPython in intro
Merge remote-tracking branch 'msarahan/ImageTutorialNotebookEdit'

Conflicts:
	doc/users/image_tutorial.rst
Merge pull request #3830 from jenshnielsen/master

Merge of #3402
Use stringIO from six

Python3 fix
Unicode issue on python3
Use open rather than file
python3 whats_new_99_spines
Add Lfunction
Python3 lightness.py
python3 demo_gridspec06
Multiple_yaxis python3
boxplot_demo2 python3
marker_reference python 3 compatible
No need to use xrange for a short loop

fixes python3