New localization and some underlaying changes.

I've added spanish localization.

I also changed the constants to properties. 
Let me explain: In the development of a project, that uses cocoapods and needed a PDF viewer, we came across with this amazing library. We rapidly notice that the constants were declared at a compiler level making them not cocoapods + git friendly (the gitignore should always exclude the pods). 
I've added properly documentation for each BOOL property and a comment with an example in the demo tab method.

Thanks for the amazing job.