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=1  turns on html output (both for main page and for iframes);
* png=1 turns on png rendering;
* iframes=1 includes iframe information to the result.

All arguments that render.html and render.png accepts are also accepted.

TODO:

- [x] tests for iframe rendering
- [x] fix iframe rendering for iframes that are created after window.onload
- [x] README

Also, 'wait' parameter, local caching and 'splash.defaults' module are sneakily added in this PR.