Adding an example.rb formula to show all features

Homebrew has many features and not all of them are documented completely in the [Formula-Cookbook](https://github.com/mxcl/homebrew/wiki/Formula-Cookbook), so often we suggest to look at other formulae. I do this myself because I forget how to do certain things :)
On the mailing list Max asked to do a reference sheet. The suggestion then was to provide an example formula (this one here passes even `brew audit` although it is totally dysfunctional).

The idea is that we keep this formula up-to-date with the recommended API to write formulae. It is a kind of cheat sheet that shows more features than the wiki currently, though some aspects are more detailed in the wiki.

Still few things missing and probably I did mistakes. Therefore I ask for review.

### Todo:
- [ ] MPI stuff
- [ ] `depends_on :fortran`
- [ ] plist stuff. I don't know much about that.
- [ ] LanguageModuleDependency other than python
...