Create MacPorts Portfile and contribute upstream

[slivingston]

Motivated by progress toward having a tulip-control port (cf. tulip-control/tulip-control#15), a Portfile should be created for polytope and contributed to the MacPorts Project. This will simplify installation for a broad audience using MacOS. Since polytope already has an entry on PyPI, I expect that creation and acceptance of this Portfile will be easily achieved.

[johnyf]

Following the conclusion of the corresponding tulip #15, GLPK is available in MacPorts. Offering polytope elsewhere than PyPI is nested (against PEP20) and is not related to linking cvxopt.glpk.

The recommended installation approach is to get non-Python dependencies with a suitable package manager (MacPorts, apt), then install Python, a virtualenv, and clone cvxopt to compile and link it to the GLPK headers installed by the package manager. This allows for frequent updates from the latest versions on PyPI, and is flat.