Code review comment for lp:~leonardr/lazr.restful/use-bleedthrough-for-methods

This branch creates the 'operation_for_version' annotation, used to publish a named operation in different ways for different versions of the web service. Currently 'operation_for_version' lets you define different versions of the methods, but it doesn't really matter, because the bleed-through stack means that only the most recent version is visible.

I made some changes to BleedThroughDict, implementing additional bits of the dict interface, so that the code in declarations.py could treat a BTD like a normal dict.

Not every annotation works with operation_for_version. For instance, the rename_parameters_as annotation directly modifies an object in the bleed-through stack, instead of putting new values in there. This means that using rename_parameters_as will modify every version of the annotation at once. In subsequent branches I'll be changing the BleedThroughDict data structure to work with this kind of annotation.