Tweaked the README to be a bit more friendly and detailed

I thought the README could use some love, so I added some details about licensing and also detailed out procedures for downloading and building the project.

In addition I added a bit to the "What do you get" paragraph and tried to tighten it all up some.
Why not link to our CI server? I guess linking people to a login page is annoying, but we could always change the link to http://ci.signalr.net/guestLogin.html?guest=1.