Moved BROWSER_TIMEOUT implementation to the gem

Paired with Tobi Gritschacher.

This adds the possibility to specify BROWSER_TIMEOUT in seconds.
This change adds support for local as well as remote browsers.

If BROWSER_TIMEOUT is set, this will affect
(1) Selenium::WebDriver's client.timeout: this specifies the time after
which Ruby's Net::HTTP will time out when a page is loaded (default: 60)
(2) Firefox dom.max_script_run_time and dom.max_chrome_script_run_time:
this specifies after which time period Firefox will consider a
JavaScript
as unresponsive and shows a modal dialog.

At the moment only Firefox is supported though there are plans to make
this work with Chromium too.

Bug: 61667
Change-Id: If55392d1fc64e1e29d0cdff14ff228718bf290ca