Add an `init` command to pod for generating a skeleton Podfile based on an .xcodeproj

My ruby is pretty rusty and my rspecfu is not strong, so I am more than open to suggestions and improvements.

I also want to expand it to include a list of pods as arguments to add the Podfile as dependencies, but wasn't sure if that should be a part of this pull request or not. I wanted to get a base implementation down.

Thoughts?