Alignment Fixes for SIMD FFT

[bsupnik]

This patch fixes unaligned memory for SIMD FFT by adding padding up to 16-bytes in sub-allocations of the various FFTs. https://github.com/mborgerding/kissfft/issues/40

[mborgerding]

I'm going to close this PR since your changes have been merged in a manual path. The PR as it was a) failing to build under Travis CI and b) did not have a test that demonstrated issue #40.

I made a few changes along this line as of cf5813a1b48744e2029293df15deb200881df927

• the existing source uses spaces, not tabs.
• assert was used but the assert.h header was not being included anywhere.
• IMO, assert is not a good way of error checking since some compilers bypass it entirely. It was used in KISS_FFT_ALIGN_CHECK which seemed like a debug artifact.
• I added a very rudimentary testsse target which now gets run under Travis CI. All it checks is that the 256x256 plan does not crash. This should be expanded to do some level of correctness testing (at least forward/inverse consistency check)

Feel like tackling issue #24? It seems to lack a champion who is willing to work on it.