OS X hets management

This is another hets related change for Mac OS X systems.
If we install hets with homebrew, there is a difference to paths we need to use (as homebrew
will install/symlink to `/usr/local/opt`).

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On another note i have created a homebrew tap which allows Mac OS X users to handle
their hets via homebrew. You can find it [here](https://github.com/0robustus1/homebrew-hets).

Here are some instructions:

- `brew tap 0robustus1/hets` will add the repository to homebrew and allow you to install hets
- `brew install hets --with-nightly` will install hets
  - it will also install the latest nightly build of the hets binary
  - it will also add a `hets-update` script to your `PATH` which will allow you to the most recent nightly build (once it changes)
  - it will also add `hets` to you `PATH` which is actually a script which will set the correct environment (like the `script` script did in the usual hets root path.


As with all homebrew projects if you perform `homebrew update` you can fetch new versions of the formula which will allow you to update to a new base version of hets (including owl-tools and the like).