On behalf of akshay@browserstack.com: using keep-alive for remote connection

(minor tweaks to patch made by lukeis)
Fixes Issue #6452
The logging level for the FirefoxDriver's native components can now be set
from the command line with the SELENIUM_LOG_LEVEL environment variable.
Valid values are ERROR, WARN, INFO, DEBUG, and TRACE. If the value is not
set, or an unrecognized value is specified, the log level will be set to
FATAL (which was the previous behavior).
Added doc strings to ActionChains and WebElement
Merge branch 'master' of https://github.com/SeleniumHQ/selenium