ENH: add support for Python 3.4 ast.NameConstant

Python 3.4 created a "NameConstant AST class to represent None, True, and False literals" (http://docs.python.org/3.4/whatsnew/changelog.html). Numpy 1.8.0b2 fails 13 tests of the following kind. The suggested change fixes those test errors on win-amd64-py3.4a2. Consider backporting this to numpy 1.8.x.

```
======================================================================
ERROR: Failure: ValueError (Cannot parse header: b"{'descr': '|u1', 'fortran_order': False, 'shape': (0,), }
\n"
Exception: SyntaxError("Unsupported source construct: <class '_ast.NameConstant'>",))
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python34\lib\site-packages\numpy\lib\format.py", line 341, in read_array_header_1_0
    d = safe_eval(header)
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1131, in safe_eval
    return walker.visit(ast)
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1018, in visit
    return meth(node)
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1025, in visitExpression
    return self.visit(node.body)
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1018, in visit
    return meth(node)
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1038, in visitDict
    for k, v in zip(node.keys, node.values)])
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1038, in <listcomp>
    for k, v in zip(node.keys, node.values)])
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1018, in visit
    return meth(node)
  File "X:\Python34\lib\site-packages\numpy\lib\utils.py", line 1022, in default
    % node.__class__)
  File "<string>", line None
SyntaxError: Unsupported source construct: <class '_ast.NameConstant'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "X:\Python34\lib\site-packages\nose\failure.py", line 38, in runTest
    raise self.exc_val.with_traceback(self.tb)
  File "X:\Python34\lib\site-packages\nose\loader.py", line 254, in generate
    for test in g():
  File "X:\Python34\lib\site-packages\numpy\lib\tests\test_format.py", line 421, in test_roundtrip
    arr2 = roundtrip(arr)
  File "X:\Python34\lib\site-packages\numpy\lib\tests\test_format.py", line 412, in roundtrip
    arr2 = format.read_array(f2)
  File "X:\Python34\lib\site-packages\numpy\lib\format.py", line 444, in read_array
    shape, fortran_order, dtype = read_array_header_1_0(fp)
  File "X:\Python34\lib\site-packages\numpy\lib\format.py", line 344, in read_array_header_1_0
    raise ValueError(msg % (header, e))
ValueError: Cannot parse header: b"{'descr': '|u1', 'fortran_order': False, 'shape': (0,), }            \n"
Exception: SyntaxError("Unsupported source construct: <class '_ast.NameConstant'>",)

```