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.
These function decorators are still causing me problems, sorry.

Decorating `moment` with `@swap2zeroaxis(['alpha'], [0])` (and commenting the `if axis is None` lines) causes the following error:

    /PyCircStat/pycircstat/descriptive.py", line 669, in moment 
    n = alpha.shape[axis]
    TypeError: tuple indices must be integers, not NoneType

So could it be that the decorator is not setting `axis=0` as you write above?

Decorating with both bootstrapping and swap2zeroaxis (i.e. 

    @bootstrap(1, 'linear')
    @swap2zeroaxis(['alpha'], [0])
    def moment(alpha, p=1, cent=False,...

gives the following error: 

    File "/Users/tomwallis/Dropbox/Projects/CircStat/PyCircStat/tests/test_descriptive.py", line 296, in test_moment_1
        mp = pycircstat.moment(data)
      File "/Users/tomwallis/Dropbox/Projects/CircStat/PyCircStat/pycircstat/descriptive.py", line 60, in wrapped_f
        ci = self._get_var(f, 'ci', None, args, kwargs, remove=True)
      File "/Users/tomwallis/Dropbox/Projects/CircStat/PyCircStat/pycircstat/descriptive.py", line 41, in _get_var
        (f.__name__, what))
    ValueError: Function moment does not have variable ci.


So it looks like the `swap2zeroaxis` decorator causes the `ci` variable to disappear (or at least, not get passed to the bootstrap decorator). Am I using these incorrectly?