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.
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Hi Tom,

I sent you (not circstat) a pull request. The two decorator problem
was more non-trivial than I thought. Anyway, it is working now and the
tests run through. Maybe you can have a final look and update your
pull request if everything's fine.

Cheers
- -- Fabian

On 11/11/2014 01:15 PM, Tom Wallis wrote:
>> +    :return:    Tuple containing three values: +
>> mp      the complex p-th moment +                rho_p
>> magnitude of the p-th moment +                mu_p    angle of
>> the p-th moment + +    Example: + +        import numpy as np +
>> import pycircstat as circ +        data =
>> 2*np.pi*np.random.rand(10) +        mp, rho_p, mu_p =
>> circ.moment(data) + +    References: [Fisher1995]_ p. 33/34 +
>> """ + +    if axis is None:
> 
> Sure, no rush. I am not going to have time to look at this for a
> few days now. The changes above are pushed to my fork.
> 
> --- Reply to this email directly or view it on GitHub: 
> https://github.com/circstat/pycircstat/pull/33/files#r20146177
> 
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