Update setup.py

including -msse3 to solve the  #error “SSE2 instruction set not enabled” when installing scikit-bio via pip
This issue has been asked and adressed here: http://stackoverflow.com/questions/26211814/error-sse2-instruction-set-not-enabled-when-installing-scikit-bio-via-pip
The solution to incluede the -msee3 flag was found here: https://github.com/rmcgibbo/mixtape/issues/35
Update setup.py

Hope that solves the flakes8 error
Merge branch 'patch-1' of https://github.com/jradinger/scikit-bio into issue-409
Only add -msse2 flag if i686 architecture

Fixes #409.
add changelog mention
Merge pull request #761 from jairideout/issue-409

Add -msse2 flag for i686 architecture