Fixed missing numpy headers in OS X build.

[kushalc]

Trying to build seqlearn on OS X 10.11 results in the following error:

$ python setup.py install
...
clang -fno-strict-aliasing -fno-common -dynamic -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/openssl/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python/2.7.10_2/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c seqlearn/_decode/bestfirst.c -o build/temp.macosx-10.10-x86_64-2.7/seqlearn/_decode/bestfirst.o
seqlearn/_decode/bestfirst.c:250:10: fatal error: 'numpy/arrayobject.h' file not found
#include "numpy/arrayobject.h"
         ^
1 error generated.
error: command 'clang' failed with exit status 1

Which is the same error solved here: hmmlearn/hmmlearn#43. This ports the hmmlearn fix to seqlearn.

[vmarkovtsev]

Nah, I wish I viewed this PR before staging my #23