Add exact solving to cobrapy

This pull request implements bindings to [QSopt_ex](http://www.dii.uchile.cl/~daespino/ESolver_doc/main.html), which will rationally solve linear programs. The binding also has the rational_solution=True parameter to return a solution consisting of only integers and fractions.

This adds also rational solving to cglpk (using the function glp_exact). This adds an additional c library dependency on libgmp in addition to libglpk, but this library can also be statically linked when creating windows and mac installers.