Caching support

This adds caching support to atosl. Basically we cache the information we read from the symbol files into a format we can read back from disk without any processing. It's sort of hacky, it could be better, but hey, it makes things faster:

```sh
$ rm -rf $HOME/.atosl-cache
$ time ./atosl -o SomeApp.app.dSYM/Contents/Resources/DWARF/SomeApp 0x12345
-[SomeAppController doneButton:] (in SomeApp) (SomeAppController.m:877)
real    0m3.463s
user    0m2.669s
sys     0m0.746s
$ ls -hl ~/.atosl-cache/
-rw-r--r-- 1 zacharyl users 9.3M Dec 11 21:48 2e8d75db26a43894b98dde4dfdb8d193
$ time ./atosl -o SomeApp.app.dSYM/Contents/Resources/DWARF/SomeApp 0x12345
real    0m1.077s
user    0m0.177s
sys     0m0.750s

Note that right now caching is the default, with --no-cache/-c to disable it. I'm not sure if this is the right thing to do, because 9.3 megs per UUID can fill up things pretty fast. On the other hand the caching will save a lot of cycles, so I dunno.