Allow a descriptor to indicate that something should be ignored and not included in the documentation

[wilkinsona]

http://stackoverflow.com/questions/32865427/document-hierarchical-json-payload-with-spring-rest-docs

At the moment there's no way to indicate the a field, link, etc should not be documented. It is possible to avoid documenting something by preprocessing the operation to remove it, but this is harder than it needs to be. I think it makes sense to add an ignore flag to AbstractDescriptor. A descriptor with the ignore flag set will prevent a test from failing due to something being undocumented, but won't appear in the documentation.