Remove manual enable/disable of IPython formatters.
Make `init_printing` toggle latex printing for classes

`init_printing` can now toggle latex text and png rendering on and off
for the relevant classes when it is re-run with a new set of options.
init_printing `png` rendering falls back to matplotlib

The `png` option on the `use_latex` flag for latex rendering will now
render latex in matplotlib if the rendering through `prewiew` fails.
Merge pull request #2609 from flacjacket/latex-printing-update

Remove manual enable/disable of IPython formatters (cont.)