OSX coreutils support

[StuartJSquires]

Tarantino uses some coreutils features that are not available in the older version of coreutils that is included in OSX (due to GPL issues). Here are the instances of this that I have noticed:

• md5sum is not available in OSX. See here and here.
• The older version of sort on OSX does not support the -V option. See here and here.
• The older version of ln on OSX does not support the -r option. See here.

There may be more occurrences of this kind of thing that I haven't found yet.

One (quick) fix is to use brew install coreutils to get the latest version of GNU coreutils. In order to avoid conflicts with the existing version of coreutils, these versions are prefixed with the letter g. This means that we would have to use if [[ "$OSTYPE" == "darwin"* ]] in the scripts to determine the OS type. I'm not sure whether or not this is a good approach, but it works.