Support source-code comments

This is what I've been working on for the last couple of days and wanted to get some feedback on style. I've made `CXComment` a single class as that's what the C API talks about, but in reality there are several classes in there, each with their own actions.

I've gone with storing the "kind" like cursors do, but it leads to a lot of ifs. I wonder if it might not be better to have multiple comment classes for ruby.