Add JavaScript execution support

This PR implements supports to execute javascript code in any of the Splash endpoints. It also add 2 new parameters to render.json (script and console) to control the display of the JavaScript output.

This is an alternative approach to a previous PR implementing JS support (https://github.com/scrapinghub/splash/pull/24) based in the feedback from @kmike and @pablohoffman. I'd like to have this PR reviewed and I am thinking closing the previous one.