Support for Jackson 2

Hello, I was scratching an itch and I thought you might want to use this code.

I have a project that uses Jackson 2.x for json serialization and I wanted to use yoga without having to import Jackson 1.x dependencies. This PR makes Jackson an optional dependency and the user can choose which version to use by including the dependency in his/her project. As long as either Jackson 1.x or 2.x is in the classpath yoga works as expected.