Mdtol for mean

This PR adds missing data tolerance (mdtol) support to biggus.mean().
I appreciate that you have followed the pattern for the `std` and `var` function docstrings but using the explicit sphinx notation for the docstring i.e. :param, :type, :rtype etc. gives a nasty (hard to read) docstring read through the `__doc__` attribute.

What is wrong with how the docstrings have been defined in all functions defined above this point?? (numpydoc docstring format)

Help on function mean in module biggus:
```Python
mean(a, axis=None)
    Request the mean of an Array over any number of axes.
    
    .. note:: Currently limited to operating on a single axis.
    
    :param axis: Axis or axes along which the operation is performed.
                 The default (axis=None) is to perform the operation
                 over all the dimensions of the input array.
                 The axis may be negative, in which case it counts from
                 the last to the first axis.
                 If axis is a tuple of ints, the operation is performed
                 over multiple axes.
    :type axis: None, or int, or iterable of ints.
    :return: The Array representing the requested mean.
    :rtype: Array
```