prefer cglpk to glpk

cglpk is now faster, more advanced, and more robust than the bindings
through pyglpk, and will be preferred to pyglpk by the automatic solver
selection process in get_solver_name. Pyglpk will remain a solver for
legacy reasons, but has been removed from the installation instructions
page.

Also, this adds the gmp license to the cglpk docstring.
Merge pull request #129 from aebrahim/solver_default

prefer cglpk to glpk