Implemented MSON section parsers

Please merge. This PR implements all related MSON section parsers.
This is completely missing out information when and what field is valid. I had to use https://github.com/apiaryio/mson-ast#type-section to recall this. This information should be captured at least in the form of a comment. 

Please add an explanation what members are used based on `TypeSectionType` and `BaseType`