fix decorator lookup by avoiding (re-)assignments to the function/class name before their execution (ticket #593)

Conflicts:
	Cython/Compiler/Nodes.py

re-allow Python values in ExprStatNode and NameNode in nogil sections if they do not need cleanup (e.g. cached builtin names or constants)

See
  http://docs.python.org/c-api/buffer.html#PyBuffer_Release
for the authoritative definition.

Conflicts:
	Cython/Compiler/ExprNodes.py
	Cython/Compiler/Nodes.py
	Cython/Compiler/ParseTreeTransforms.py
	Cython/Compiler/Symtab.py

error tests for nonlocal, fixes to properly detect these errors (Python gives warnings for redefs, but I find errors more appropriate)

moved 'nonlocal' generator test over to generator test module to make all nonlocal tests compile in Py2.4