2-D Periodogram

Translation of a matlab function I had in the style of the current periodogram function. I don't think there is anything in matlab's DSP to compare it to for testing. It is pretty fast for reasonably sized matrices:
```
# 100 iterations
periodogram (N=1024x1024), 
elapsed time: 1.872988929 seconds (841653880 bytes allocated, 8.75% gc time)
```

EDIT: This is a radially summed spectrum (and is commonly used in fluid dynamics literature), like [this](http://www.mathworks.com/matlabcentral/fileexchange/23636-radially-averaged-power-spectrum-of-2d-real-valued-matrix) , but replacing `nanmean` with `nansum`. Both give the same result for test cases.
I appreciate your effort here, but I'm not totally sold on the utility. For plotting, it seems like it would be more useful (and less code) if the Periodogram2 object just carried around two Frequencies objects.