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()

cosolidate exception messages
cleanup of utility code

changed more exception messages to what Py3 raises
some cleanup

use argument exception messages from Python 2.6
TODO: handling of keyword-only arguments seems to require more work
TODO: clean up helper functions (currently somewhat redundant)

changed Python C-API usage test case to test includes from Cython/Includes/, currently fails: are cimports in .pxd files not re-exported?

some EncodedString name fixes for function names