cglpk: bugfix to LP file writing with python 3
Add exact solving to cglpk

This is controlled by the exact=True parameter. However, this requires
an additional dependency on libgmp (which can also be statically
linked).
add bindings to QSopt_ex aka esolver

The solver uses cglpk to construct the problem and write out an LP, and
the command line interface to esolver is used to solve it. Esolver can
use rational math to solve an LP exactly.
test esolver on travis