Whitespace
Use more general 'Sequence' and 'Mapping' for type checking...

where appropriate, because duck-typing is more Pythonic and will allow for
extended uses of Cerberus.
Add documentation about exactly what type checking is now performed...

when specifying 'dict' or 'list' data types
Docstring for validate_schema
Merge branch 'ch3pjw-relax_type_checking'