supporting full web page screenshots in sahi

[sarabsandhu]

I would like to have support for taking full web page screenshots in sahi through api where I can pass the name of image and location where I would like to save it. This will enable me to compare webpages from one version to another. I can do the same through casperjs (http://casperjs.org/) and phantomjs(http://phantomjs.org/). Casperjs provides navigation api for phantomjs. It also provides access to other api like (capture,captureSelector) to capture high quality images of the web page. Currently, I can use phantomjs as headless browser in sahi but there is no API exposed in sahi yet. I would love to have phantomjs api exposed in sahi.