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.
Can I get the `swap2zeroaxis` decorator to return `axis` to the body of the function? You can see that the argument `axis` gets used at several points later in the function (e.g. `theta = mean(alpha, w=w, d=d, axis=axis)`). If the decorator can't return `axis` back into the function, how can I both treat the default `axis=0` (plus ravel) and also allow the user to specify an axis if desired?