Ticket 65

cleanup of arg parsing code, faster string comparison in __Pyx_SplitKeywords(), some minor exception fixes

some code cleanup and beautification, fix error message to match Python when passing too many positional arguments into a function with required keyword arguments

removed some redundancy from arg parsing helper functions (and some of the helper functions)
general cleanup and performance improvements (now a lot faster for common kw passing casese)

optimisation: store keyword argument names as Python strings to avoid string creation within PyDict_GetItemString()