periodograms for 2-d signals

2-d periodograms, radial averaged and summed periodograms, Frequencies2 type,
fftfreq2 function, nextfastfft for tuples
Frequencies2 removed

two Frequencies objects used instead
fftshift for Periodogram2 added + tests
v0.2 support removed
Merge pull request #45 from gummif/per2d

2-D Periodogram