Added podspec and license file

[rhodgkins]

Added a pod spec for version 1.0.0.

A 1.0.0 tag needs to be created before this can be pushed to the spec repo.

I'm more than happy to maintain the podspec if you don't want to or have the time...!

Thanks for the library :+1:

[rhodgkins]

Also changed to use instancetype for init methods.