glpk_solver parameter improvements

The following changes were made:
 - raise an exception when passed an unsupported option.
 - Use tolerance_feasibility instead of tolerance_optimality for tol_dj
   and tol_bnd
 - support for lp_method as strings
cglpk supports lp_method and verbose options

also warns for unsupported threads and mip_gap_abs
improve cplex parameter handling

When setting parameters, the cplex solver will now convert strings
encoding the cplex constant to the constant itself. For example, setting
lp_method="auto" will cause execution of
lp.parameters.lpmethod.set(lp.parameters.lpmethod.auto)

Additionally, an invalid paramter will raise a ValueError. Finally,
verbose is handled as an option to allow output (silenced by default).
gurobi parameters updated

Now supports lp_method strings and verbose.