Add formula for realpath.

realpath-osx is a simple program for OS X which recreates the behavior of the realpath command on other unix-like systems, such as Linux.

```
jmhobbs@venera:~ ✪ realpath .
/Users/jmhobbs
jmhobbs@venera:~ ✪ ln -s /usr test
jmhobbs@venera:~ ✪ cd test
jmhobbs@venera:~/test ✪ realpath .
/usr
jmhobbs@venera:~/test ✪
```
`system "#{bin}/realpath", "."