Fix installation

With Cython extension StructuredPerceptron.fit is 2.5x faster than with vectorized numpy for my data, thanks! 

But installation now requires numpy at build time, so without proper headers it fails. A simplified version of scikit-learn's setup.py numpy code is in this PR. I'm not sure what other numpy-related stuff does in scikit-learn setup.py; numpy.get_include() seems enough for seqlearn.