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. 
You shouldn't write into another formula's Cellar and other formulae shouldn't need to know the version number. Symlinks general solve both these problems. Setting `HOMEBREW_PREFIX` is OK but only in the cases where no files are installed into `HOMEBREW_PREFIX` rather than the Cellar.