Added "Cancel button" and a library project

[dodikk]

Added a cancel button.
Added a static library project for better code reuse experience

[matej]

Thank you for your contribution, but I won't be pulling in those changes at the moment.

Here are the main reasons:

• I would like to avoid keep this component to two files only and avoid any resource files like bundles
• iOS 7 is moving away from bordered buttons
• the code style doesn't quite match the rest of the class
• there have been significant changes to the project structure in the meantime, including a library target

Neither the less, having some sort of cancellation support has been a frequently requested feature so I'll definitely keep this in mind.