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. 
Given installing ` ipe ` automatically creates those directories, is it necessary to do this bit at all? Unless you're worried about the ` ipe ` in ` lib ` being a symlink back to ` ipe `'s Cellar, which under normal Homebrew build processes it is?