Added specific errors for most validation errors
Increased test coverage by a tiny bit...
Changed comments to docstrings
Added fix for custom messages error throwing.
Added fix for custom messages error throwing.
added Remove marker and new extra keys treatment option
fixed the Msg error and added more Invalid subclasses to handle Is* functions
updated mapping sort priorities to avoid clashes between remove and other schemas
added more Remove tests
Merge pull request #82 from sirfz/add_remove_capability

Add Remove marker and option for extra keys to be excluded from output
Merge pull request #81 from bdejong/master

Added specific errors for most validation errors