Make Cython a proper setup_requirement

[mjpieters]

Currently, if a user doesn't have Cython installed, trying to install this package results in an error:

Collecting tsne
  Using cached tsne-0.1.7.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-9w8mswlw/tsne/setup.py", line 18, in <module>
        from Cython.Distutils import build_ext
    ImportError: No module named 'Cython'

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-9w8mswlw/tsne/

Please specify Cython in setup_requires, and leave handling compilation of .pyx files to setuptools; as of version 18.0 setuptools supports this natively:

setup(name='tsne',
      version=versioneer.get_version(),
      cmdclass=versioneer.get_cmdclass(),
      author='Daniel Rodriguez',
      author_email='df.rodriguez@gmail.com',
      url='https://github.com/danielfrg/py_tsne',
      description='TSNE implementations for python',
      license='Apache License Version 2.0, January 2004',
      packages=find_packages(),
      ext_modules=ext_modules,
      setup_requires=['cython'],
      install_requires=required
)

(dropping the Cython imports and the two lines that use those imports)