remove all sources and update README

This PR removes all sources and updates the README to state that three20 is not maintained anymore.

It seems there are still new iOS developers that pick up three20 from somewhat old blog posts or tutorials and then ask questions about it on stackoverflow. By removing all sources and updating the readme it should become clear that three20 is at the end of its lifetime.

I'd propose to merge this PR into development-branch, after that merge the new development branch into master. 
Optionally, one could do a final release (merge development to master, tag 1.0.12). If someone feels like maintaining or merging one of the open PR, this commit could ofc be reverted on development later.

What do you think?