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. 
What is this? `cgal-ipelets` formula? Not exactly, not without setting environment variables. Without changing this line, `ipe` will look for ipelets under `/usr/local/Cellar/ipe/...`, which is not where any other package would install ipelets. If you run, `ipe -show-configuration`, you'll see the problem.