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.
Fixes reaction deletion/removal

This pull request is around the functions Model.remove_reactions,
Reaction.remove_from_model, and Rection.delete.

These fucnctions now all have a flag for removal of orphan
metabolites/genes. Before, this behaviro was unspecified and unclear.
Moreover, this behavior is also tested under the unit tests.

Previously, the function Reaction.delete had bugs which prevented it
from working correctly, and was not under unit testing. This has now
been corrected.
warn and skip non-ascii gene_reaction_rule in sbml