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

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)

fix keyword arguments in function calls: must be unicode in Py3 and strings in Py2, just like identifiers

tons of fixes for the test suite: docstrings must be unicode strings to run in Py3's doctest (obviously...)

--HG--
rename : tests/compile/withnogil.pyx => tests/run/withnogil.pyx