More Swifty improvements

Following up on the discussion in #172, I made capitalized identifiers highlight as `Keyword.Type`. Also fixed a few edge cases and simplified the code a little bit.

I tested the changes on some (quite complex) files from my Swift project and I'm quite happy with the result :)

Unless I broke something on the way that I didn't notice, I think we should be set for a while ;)

/cc @ole
I'm actually not a super big fan of this style - if you could put parentheses around the argument that'd be great.  Also, I'd rather continue matching `#{id}` and casing off the match if it's possible, rather than constructing giant regexes dynamically.