weighted mean
A "weights" labeled parameter in the df.mean method would be extremely useful. In numpy this functionality is provided via np.average instead of np.mean which I'm assuming is how similar functionality would be added to pandas.

ex requested feature:

```
> a = np.array([[2,4,6],[8,10,12]])
> w = [.5, 0, .5]
> np_avg = np.average(a, weights = w, axis=1)
#output ->  array([ 4., 10.])
> pd_avg = pandas.DataFrame(a).mean(weights = w, axis=1)
#desired output -> series with entries 4 and 10
```

If this is a desired feature I'll complete this and submit a pull request if desired. If this is somewhere already I've overlooked, my apologies, I've tried to look thoroughly.
