New features: screenshot, open URLs in new window, query for +windowFocused

[digitalsignalperson]

• Added bt screenshot to get a screenshot of the current visible tab. Also returns the api prefix, window id, and tab id with the base64 png data (#106)
• Modified bt open so if window_id is specified as "0", a new window will be opened. Multiple URLs provided to stdin will all be opened in the new window (#39)
• Added a query option for +windowFocused or -windowFocused to detect if the browser application is being used or not

also the first commit updates requirements.txt as a work-around for #105

[rolflobker]

Such a shame this repo is not maintained anymore Awesome PR's like these should be merged into a more maintained version.