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.
cplex and mosek accept other numeric types

For example, decimal.Decimal
Merge pull request #129 from aebrahim/solver_default

prefer cglpk to glpk
small cleanup to cglpy.pyx and solver test

Solver testing now tests for an optimal solution status for each test in
the change_coefficient testing. This testing code is also now pep8
compliant.