Add action to copy chromedriver.
Create dist for chromedriver and upload it.
default_app: Don't quit when started as web driver.
Make our user agent string follow standard.
Pretend to be Chrome by default.

This is used to cheat client web drivers.
docs: Document how to use chromedriver.
docs: use => using
Merge pull request #652 from atom/chromedriver

Ship chromedriver and add docs on how to use it