updated Xcode command line tools install procedures, fixes #376, fixes #352

I was sitting in the volunteer meeting for RailsBridge NYC and thought that this might help for installfests.

The last time I did one of these workshops, everyone was trying to download Xcode at the same time and it made the internet connection crawlllll... going for just the command line tools will help with that and will save space on the students' computers.

I think this is a simpler install process anyway, and it will work for the 10.9 and 10.10 people who are confused when they can't find the option to install it from Xcode.