option callback on value change, use an option to control use_inf_as_null
I'm not married to the "mode.X" option key, but be aware that the dynamic generated 
docstrings for the options API favour a small number of top level keys with multiple keys
under them and so i consolidated `test.interactive` into `mode.sim_interactive`.
Some sane taxonomy needs to be established early on to keep things tidy,
Be an option-naming fascist I say.

http://martinfowler.com/bliki/TwoHardThings.html

related: #1919

@wesm , just want to make sure this and the other 0.10 PR's won't get left behind,
so we avoid back-compat issues.
 don't worry, i'll definitely give fair warning before cutting 0.10 final...
