added new Get Started Page, updated links and css

Created a Get Started page. I consolidated the instructions in the http://bundler.io/v1.3/rationale.html, the "Get started-installing Bundler" directions and the "Simple work Flow" directions. They all repeated but with a few extra tidbits of info. Andre gave me the go ahead to do this on Friday.
I also added the link to the sidebar and added a bit of css styling.
I shortened the index page and added links to the Get Started page.