Add framework targets for iOS, watchOS and tvOS

[louisdh]

The project in its current state specifically says "for iOS", would it be ok to change this and add framework targets for iOS, watchOS and tvOS?

[lrtitze]

I just got around to merging the iOS framework pull request from @an0. If you have the cycles at any point to add watchOS and tvOS feel free to go ahead. Thanks for your contributions.