Added function `moment`

This is my first contributed function (i.e. please someone check thoroughly). I've checked the basic functionality against circstat (matlab), and added a number of tests. 

Questions and issues:

1. Currently the arguments `w` and `d` only alter the behaviour of this function if `cent=True`. Should they be doing something additionally in the body of the function?

1. I return the values in a tuple. I notice that most of our other functions return only single values, making them more compatible with e.g. the bootstrap decorator.

1. Further to this: does it make sense to include bootstrapping for the moments, and if so, how should this be done (a simple attempt broke because this function outputs a tuple).

Assuming this function is acceptable, I plan to do skewness and kurtosis over the weekend.
I know that no other function uses it. I wrote it later and didn't convert the other functions. The trick of the decorator is that it reshapes the input array depending on what axis argument is given such that the desired axis is at axis=0. Then it also sets the axis to be 0. If you try with a 2d array and axis=1, the array should be transposed and axis should be 0.