Make numpy an optional dependency

to install with fast HyBi support, use smth like

`pip install websockify[fastHyBi]`

setup.py

@@ -34,7 +34,9 @@ setup(name=name,
 
       packages=['websockify'],
       include_package_data=True,
-      install_requires=['numpy'],
+      extras_require = {
+          'fastHyBi': ['numpy']
+      },
       zip_safe=False,
       entry_points={
         'console_scripts': [