OS X 10.8 (Darwin) Support

Adding support is a little tricky as a fresh install of OS X will not have the "Command Line Tools for Xcode" installed by default. I dislike adding an option switch, as it seems, unclean. However, not being able to automate the install of salt on OS X is even more annoying. At least to me. If there is a better way of passing this information I'd be fine with changing it.

I'm not 100% certain that the script is still pure sh despite `sh -n salt-bootstrap.sh` returns no errors due to sh being bash in sh compatibility mode.

When testing it might be best to installing with `./salt-bootstrap.sh git v0.13.3` as there appears to be a few issues in v0.14.0 and develop at the time of this commit. I think the install process works fine, and the keys are register with the salt-master however I am not able to do a test.ping successfully.

Finally, I'm also made a pull request to include the launched plist files for the services so until/if that pull request is accepted the services won't install/run.