#16 Interface to export and refactoring

[FlexibleCoder]

Since that the export to pdf (the most requested and used) was the target of refactoring to become as reliable as possible with the preview, the engine will be target to a new refactoring with regard to exports in general, ie, instead of the code is mixed, will have a interface that defines the methods of exporting and each export will implement the said interface to organize the code to became more easier to maintenance the current exports formats and to create new in the future.