Add newaxis constant for use in array indexing (#414)

* Add `newaxis` constant

* Update ToC

Author: kgryte

spec/API_specification/constants.rst

@@ -22,4 +22,5 @@ Objects in API
    e
    inf
    nan
+   newaxis
    pi

spec/API_specification/signatures/constants.py

@@ -15,11 +15,16 @@
 IEEE 754 floating-point representation of Not a Number (``NaN``).
 """
 
+newaxis = None
+"""
+An alias for ``None`` which is useful for indexing arrays.
+"""
+
 pi = 3.141592653589793
 """
 IEEE 754 floating-point representation of the mathematical constant ``π``.
 
 ``pi = 3.1415926535897932384626433...``
 """
 
-__all__ = ['e', 'inf', 'nan', 'pi']
+__all__ = ['e', 'inf', 'nan', 'newaxis', 'pi']