Fix info_overlay resolution

I picked up a little linear algebra a couple months ago, and between that knowledge and the motivation of a new project, I've managed to work up what I believe is a solution to issue #14, which I posted back in April. 

There's the FGD parsing side of things, where the parameter definition regex didn't pick up "readonly" properties, and then there's a few special case needs that info_overlay has. The different capitalization Valve uses in base.fgd, where some things are "vector" and others "Vector", doesn't seem to consistently indicate a difference, and only through trial and error did the answer make itself clear.

I've got a test map I used to check my results, rotating a func_instance all over the place and nesting it two levels, and I can commit it if you want, but I didn't want to clutter up the repo with these things any more than I have already (I didn't mean to be pushy with those vecline tests way back when, I just wanted to be thorough).

Let me know if you want any changes and I can get right on them!