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.
So here's a funny thing. It seems that adding the decorators means that the python debugger (i.e. the `pdb.set_trace()` command) and also print statements no longer work. Consequently I haven't been able to test this line (because the decorators are now working). I'm not sure what we can do about this.

Regarding this line: I think using `size` is fine, because the `swap2zeroaxis` decorator should ravel everything anyway, so size *is* the length of the `axis` we're interested in. Correct? The line is used to tile the mean along the axis so that each alpha value has the mean subtracted (i.e. is centred).