[Python] stop instantiating colander schemas at every use site

[suntzu86]

From my comment on #291

lots of colander examples appear to instantiate schemas immediately after definition so that they're only instantiated once. We do a mix of A) instantiate as class variable or B) instantiate at use site. It'd be good to establish what standard colander style is and follow that. The choice here could be relevant to issue #336 b/c it may expose more instances of that problem.