[WIP] render.json endpoint

render.json endpoint is introduced in this PR.

It returns basically the same output as old iframes.json endpoint + a base64-encoded screenshot.
It accepts the following GET arguments:
* html=0  turns off html output (both for main page and for iframes);
* png=0 turns off png rendering;
* iframes=0 limits information to be only about the main page.

All arguments that render.html and render.png accepts are also accepted.

TODO:

- [ ] tests for iframe rendering
- [ ] fix iframe rendering for iframes that are created after window.onload
- [ ] README

I based this PR on https://github.com/scrapinghub/splash/pull/11 because it is hard to run tests otherwise.