Upgrade for numexpr 2.1rc3

This closes PyTables/PyTables#24 and PyTables/PyTables#117 and PyTables/PyTables#199.  

Additionally, the ``_equiv_expr_node()`` function in conditions.py (see below) should be considered for inclusion in numexpr.  It tests if two NodeExpressions are equivalent.  This is needed since the NodeExpression.__eq__() method now returns another NodeExpression rather than a bool.  If this function is moved over and made part of the numexpr public API, then we will remove this from PyTables.

Lastly, when I run the tests with numpy master I see some DeprecationWarnings that I believe are coming from numexpr (see below).  I don't think that these should prevent this PR from being merged in.

*function:* 
```python
def _equiv_expr_node(x, y):
    """Returns whether two ExpressionNodes are equivalent.  This is needed
    because '==' is overridden on ExpressionNode to return a new ExpressionNode.
    """
    if not isinstance(x, ExpressionNode) and not isinstance(y, ExpressionNode):
        return x == y
    elif type(x) is not type(y) or not isinstance(x, ExpressionNode) \
                                or not isinstance(y, ExpressionNode) \
                                or x.value != y.value or x.astKind != y.astKind \
                                or len(x.children) != len(y.children):
        return False
    for xchild, ychild in zip(x.children, y.children):
        if not _equiv_expr_node(xchild, ychild):
            return False
    return True
```

*warning:*
```bash
/home/scopatz/.local/lib/python2.7/site-packages/numexpr-2.1_rc3-py2.7-linux-x86_64.egg/numexpr/necompiler.py:746: DeprecationWarning: using `oa_ndim == 0` when `op_axes` is NULL is deprecated. Use `oa_ndim == -1` or the MultiNew iterator for NumPy <1.8 compatibility
  return compiled_ex(*arguments, **kwargs)
```