Windows support

[btrask]

We're using libuv so we get a lot for free. The main problems that I can think of:

• We do path concatenation using / as a hardcoded separator
• The Markdown parser doesn't use libuv
• The LevelDB wrapper makes one call to unlink(2), when it could probably get by with using LevelDB's env object
• We have a couple random calls like raise(3) and setrlimit(2)
• We rely on GCC/Clang features, most notably Objective-C (if GCC or Clang is passable on Windows, I'd be okay with requiring it)