waitFor and waitTimeout : issue #39

Add the waitFor and waitTimeout parameters. waitFor will be evaluated periodically within the phantomjs browser; if specified, screenshot-as-a-service won't take the screenshot until waitFor evaluates to true. Example: "function() {return Config.get('loaded') == true}" -- note, you must URI encode it. waitForTimeout specifies a maximum time to wait before giving up and rendering the screenshot anyway.

I'm sure there's more I have to do, like update the readme and example config. If you can take a look to see if it's functionally sound beforehand I'd appreciate it.