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.
These do not seem necessary, since Frequencies2 returns tuples and `+` and `-` are not defined for tuples.