Adds osx webcam and record_mic post modules

Uses the same approach from #2240 to call objc functions in OSX's native libs using Ruby's ::DL library. The  incompatibility between ruby 1.8 DL and 1.9 DL is made very obvious here by all my patches. I've moved the shared client-side code into a mixin (which I just put in `/lib` - is there a better place for this?), so I can update my keylogger story after it lands to utilize the mixin and dry things up. Tested on OSX 10.8 and 10.7.

Testing is straightforward:
- [x] Gain a session on OSX, use the webcam post module, verify that LIST, SNAPSHOT, and RECORD actions all work as expected
- [x] Gain a session on OSX, use the record_mic post module, verify that LIST and RECORD actions all work as expected