cglpk: add function to write problem to LP file
quadratic_component is an optional keyeword arg

No longer pass quadratic_component=None to all solvers by default and
force them to implement code to ignore it if they do not support
quadratic programming. When specified, it is still passed on.
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
Merge pull request #126 from aebrahim/exact

Add exact solving to cobrapy
Save genes in JSON and mat files.

Still not passing unit tests with SBML io.