glpk implementation written in cython

This requires the improved solver interface from #62.

This solver is written in Cython and uses the c API for GLPK directly.

Advantages:
 - pyglpk uses deprecated/removed API's, and has not been updated to use new ones
 - solver instances can be pickled
 - allows multithreading by releasing GIL during solve (not sure if this is useful)
 - it can use newer (and therefore faster) versions of glpk