Added save screenshot method

[rpapallas]

to allow clients to programmatically save screenshots to a desired path.

[rohanpsingh]

Thanks for the PR. I have a few comments:

• The save_screenshot method should probably only return the image as an array. Writing should be handled by the user code.
• I am planning to implement a record video functionality soon. Then I may have a better idea on how to best integrate both these features.
• I'm afraid that adding too many new API calls that the user has access to would require a much better documentation than what we have now. The code would probably also need to be refactored to clarify which methods are meant for the user.

It should be safe to merge this though, but let me first implement the record video functionality .

[rpapallas]

I agree, you might want to look into readthedocs for publishing docs, which you can publish as a GitHub page.