Add the list of nodes within Parser::Meta

This pull request addresses #121. Unparser and mutant do not have tested support for 2.1 node types (if there are any). Before merging this one we should verify no 2.1 specific nodes are missed.

Unparser successfully round trips rubyspec, so no 1.8 - 2.0 nodes should be missing from the list. I currently do not have an idea how we can avoid superfluousness items.

Nevertheless I think this node list can be included now, and will be used by two actively maintained tools (unparser, mutant) I'll push any correction that is needed directly to master.