Create ASL capture module.

A number of notes:

* The code is basically modified from the link I sent in the previous issue. Not a whole lot has changed.
* I've added TODO: comments where your attention is required
* It's not yet integrated with the rest of the Lumberjack framework as it's not yet clear the best way to do so. Also, I'm not sure if it makes sense to forward the messages on to DDLog since if you have a TTY logger or an ASL logger installed, then they will end up getting duplicates and you can potentially end up in an infinite recursion if you have an ASL logger. Really only makes sense for the file logger, or perhaps custom loggers people may write. Not sure the best way to proceed.
* I'm not that experienced with GCD so I can't attest to the stability of the code. I haven't tested start/stopping the ASL capture module.
* My Xcode has tabs set to 2 spaces, which is different than yours apparently. Sorry :-)