Ignore unmapped properties in YAML

Given a YAML file that contains more information than the object deserializing it, the current implementation will throw an exception because there is not a corresponding object property for all properties in the YAML file. This fixes the issue (or at least starts to address it).

It seems that there [would be some interest][interest] in this solution.

Let me know what else I need to do to integrate this code.

[interest]: http://stackoverflow.com/questions/20250608/yamldotnet-deserialization-throwing-exception-when-yaml-document-has-a-field-not