defer_permissions option for permission issues with sshfs on osx

As mentioned as a side note in #42, there are permission issues in OSX. `-o defer_permissions` is an OSX only option and will cause errors on other systems where it is not available.

This verifies if the system is OSX with `uname`, then adds the option if it is.

If you want to test, try reading a php.ini file without the option. Access will be denied because of invalid ownership. Once the file system is mounted with the option, you will be able to read php.ini.