include Response class in default simplate context

[chadwhitacre]

Suggested by @techtonik at gratipay/gratipay.com@4d7d0dc#commitcomment-8820613. It's definitely a common error, NameError: name 'Response' is not defined. This usually lies along error paths, and error paths are undertested as a rule, so it's a bug that lurks waiting to mask real errors.

[techtonik]

There seems to be response object as well. Might be useful to move Response API there.

And if there is request object, move path API there.

[pjz]

This was done when we moved to passing around all of algorithm state, yes? Oh, wait, no, they're wanting the class, not the default response. Uhh... I don't think we want to do this. We're moving toward being more specific about context imports, not less.

[chadwhitacre]

We're moving towards more explicitness with simplate contexts: #30. Closing.