Improve grouping of zeros with poles for SOSFilters

SOSFilter conversion now checks that complex poles and zeros form
complex conjugate pairs, and I've tweaked the algorithm for pairing so
that it matches MATLAB and added tests.
Fix a type instability issue in SOSFilter conversion and add asserts
Don't cancel equal zeros/poles on ZPKFilter construction
Merge pull request #67 from JuliaDSP/sjk/sos_pairing

Improve grouping of zeros with poles for SOSFilters
Support fftfilt/firfilt along columns

Also reinstate fftfilt/firfilt tests, which it seems like I
accidentally removed in #64
Faster FIR filtfilt

For FIR filters, it is better to use firfilt than filt since it ensures
an appropriately fast algorithm is used. Additionally, the filter can
simply be convolved with itself so that it is not necessary to process
the data in the forwards and backwards direction. There is no need to
compute the initial state because it has no effect on the result after
the signal is padded.

This algorithm could still be made more efficient with an in-place
variant of firfilt that can save the FFT of the filter.
Estimate speed of filt vs. fftfilt based on length of whole array
Bug from previous commit
Merge pull request #68 from JuliaDSP/sjk/fir_filtfilt

Faster FIR filtfilt
2-d windows
Allow sampling frequency to be non integer value
Re-enable testing for julia-releases now that it is v0.3
Define Base.time instead of time in Periodograms

Fixes #72
Fix dpss antisymmetric tapers if first element = 0 to machine precision
Merge pull request #73 from codles/master

Allow sampling frequency to be non integer value
Merge pull request #71 from gummif/win2d

2-d window functions