CGAL ipelets

This pull request adds cgal-ipelets formula (ipelets which are included with CGAL, but not installed by default). It also fixes a small, but inconvenient bug in the ipe formula. 
Ah, yes:

```
Ipelets: /Users/Dominyk/.ipe/ipelets, /Users/Dominyk/Library/Ipe/Ipelets, /usr/local/Cellar/ipe/7.1.5/lib/ipe/7.1.5/ipelets
```

Alright. As long as we're not suddenly going to start dropping ` ipe ` directly into ` /usr/local ` that's fine, I think.