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. 
That feels awfully hacky. I'm new to Homebrew, but I'd imagine that `ipe` directory in the cellar should contain only Ipe's own code. Imagine a different package that provides some libraries, executables, and also some ipelets. Should it scatter stuff all over the cellar? That seems strange to me.