cocoapi

[Why] When we perform `pip install https://github.com/pdollar/coco.git#subdirectory=PythonAPI`,
it requires `matplotlib` and `cython` package.

Although we put cython in our requirements.txt, sadly `pip install
-r requirements` doesn't follow the speficied order according to
https://stackoverflow.com/questions/5394356/how-to-specify-install-order-for-python-pip

I wanna change setup.py to install coco python api along with cython.

[How] According to https://stackoverflow.com/questions/37471313/setup-requires-with-cython,
we can use `setuptools` to speficy cython as setup_requires and it will
deal with `*.pyx` automatically