fix ticket #578 by working around CPython bug 9834: crash in Py3.[0-1.2] when slicing non-sliceable objects

store docstrings directly in the dict instead of looking them up at module init time
=> much faster, a lot less code, fewer redundant string constants (duplicate strings are unified anyway), and just as good, as docstrings of Cython functions/methods can't currently be changed anyway

clean up directive decorator parsing, support boolean directives as plain names, support directives on cdef classes

Py3 install fix: keep original Py2 source modules of Cython alive during installation when replacing them with 2to3-ed versions - we still need the build_ext stuff