Improve setup.py to work on a machine without numpy or cython

[yuyu2172]

Related #325

The following commands will successfully install ChainerCV.

docker pull ubuntu
docker run -it ubuntu /bin/bash
# in docker session
apt-get update
apt-get install git
apt-get install python-pip
pip install -U pip
pip install pillow
pip install Cython
git clone -b setup-without-numpy https://github.com/yuyu2172/chainercv 
cd chainercv
python setup.py install

[yuyu2172]

I updated the code to work on a machine without Cython if .c is in the directory.
This is the case when pip is used.
I tested in an environment where I copied .c file to the repository from Github, and run the previously posted installation commands without Cython installed.

docker pull ubuntu
docker run -it ubuntu /bin/bash
# in docker session
apt-get update
apt-get install git
apt-get install python-pip
pip install -U pip
pip install pillow
# pip install Cython
git clone -b setup-without-numpy https://github.com/yuyu2172/chainercv 

### get .c
apt-get install wget
wget https://pypi.python.org/packages/d9/5b/0dc17559a11b49a93868714ef7228b67b98f800e9faec940857c759d27bb/chainercv-0.5.1.tar.gz#md5=23d188d95854fb781a01698a6f11f447
tar xvzf chainercv-0.5.1.tar.gz
cp chainercv-0.5.1/chainercv/utils/bbox/_nms_gpu_post.c chainercv/chainercv/utils/bbox/

cd chainercv
python setup.py install

[Hakuyume]

@yuyu2172 Could you remove the following lines from the document?

# If Cython has not been installed yet, install it by a command like
# pip install Cython

https://github.com/chainer/chainercv/blob/master/docs/source/index.rst#pip

[Hakuyume]

We cannot remove this line because this repository does not contain *.c files. Without this line, Travis CI fails.

[yuyu2172]

Thanks.

[Hakuyume]

LGTM