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 would vote for not using three output arguments. In that case  (as you mentioned) the bootstrap decorator can easily be used. The angle and the absolute value are easily computed anyway. As an alternative, we can write a wrapper function.