Added Guile.podspec file

This allows one to add the `Guile` repository directly in a `Podfile`. For example: `pod 'Guile', :git => "https://github.com/gilt/guile.git"`

This will probably have to be added to the `0.1.3` tag, and also be updated on CocoaPods so we don't have to point to this repository :smiley_cat: 

:shipit: