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

--HG--
rename : tests/run/tnumpy.pyx => tests/run/numpy_test.pyx

Removed use of <BLANKLINE> doctest directive.