Rotates user agents from the 3 browsers, Chrome, Safari, and FireFox.
get_agent = Get_Agent()
random_agent = get_agent.Chrome()
Returns:
a random agent: "Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.8.0.12) Gecko/20070530 Fedora/1.5.0.12-1.fc6 Firefox/1.5.0.12"