Set default chrome path based on the os.

Added code for finding the default chrome binary path based on the os
running. For Windows, will try the following paths in this order,
'%LOCALAPPDATA%\Google\Chrome\Application\chrome.exe', 'C:\Program
Files\Google\Chrome\Application\chrome.exe', 'C:\Program Files
(x86)\Google\Chrome\Application\chrome.exe'. For OS X, will only try
'/Applications/Google Chrome.app/Contents/MacOS/Google Chrome'. For
Linux, '/usr/bin/google-chrome' and '/usr/bin/chromium-browser' will be
tried.