Enumerator validation

Adds support for optional enumerator validation in the schema file. Practically, this means that an option can be one of a number of discrete strings. These are added in a third portion of the returned tuple from the schema parser. This may break your code if you rely on only unpacking two elements from the schema parser, see the unit tests for an updated implementation. This version is now the current on pypi, so if you would like to keep using the old implementation, specify yamldoc==0.1.3 to avoid the new complexity.