Implement fftshift for Frequencies

Fixes #56
Initial add of filtfilt function and tests
Tidy filtfilt code
Use eye instead of I for julia v0.2
Test filt_stepstate with a[1] != 1

Currently the test fails
Rescale output
Implement fftshift for periodograms and spectrograms
Merge pull request #57 from JuliaDSP/sjk/fftshift

Implement fftshift for Frequencies
periodograms for 2-d signals

2-d periodograms, radial averaged and summed periodograms, Frequencies2 type,
fftfreq2 function, nextfastfft for tuples
Move filt! implementation to backwards compatability file
Merge remote-tracking branch 'upstream/master'
Reverse in place
Tidy test code
Test for 2d filtfilt
Remove backwards compatability
Add support for 2d arrays
Fixed 2d filtfilt. Exported coeffs
Removed redundant comments
Faster filtfilt

By my benchmarks this is approximately 10x faster
Require Julia 0.3, and remove 0.2 support code