Feat: Add Screencasting of the browser, save images. v0.15 compatible with Cuprite

[singhprd]

Work in progress.

To use this:

page.driver.browser.page.start_screencast(save_dir)
page.driver.browser.page.stop_screencast

This only saves images, it's not true video/screencasting but that is actually what Chrome provides.
This could become a 'real' screencast by streaming the images received from Chrome to something that combines them into a video (ffmpeg), but that would require much more complication in this Gem - and can just be done by a user later.

They should be very helpful for viewing and debugging tests.

Taken from this issue discussion: #354

Todo:

• Add tests
• Add configuration for screensharing: https://chromedevtools.github.io/devtools-protocol/tot/Page/#method-startScreencast