Update setup.py to tie in numpy includes location.

Build of statsmodels==0.5.0 errors out in jenkins build:
test_algo_crash (integration.test_arbiter_deluge.TestArbiterDeluge) ... /mnt/jenkins/.pyxbld/temp.linux-x86_64-2.7/pyrex/statsmodels/tsa/kalmanf/kalman_loglike.c:340:31: error: numpy/arrayobject.h: No such file or directory

Update setup.py to include the location of numpy header files.