Add install and extra requires

[godber]

I have added install_requires which will provide dependencies for all installs and extras_require which should support installations that install extra dependencies.

So

• pip install openvr will install openvr and numpy and PyOpenGL, let me know if I've missed any global dependencies
• pip install openvr[glwf] - installs glwf
• pip install openvr[PyQt5] - installs PyQt5
• pip install openvr[wx] - installs wxPython

I should probably add this info to the README ... also PySide seems dead, I can add it but not conditionally for older pythons (as far as I understand, maybe it is possible). I don't have VR on linux handy to test SDL2, I would have added it if it appeared to be pip installable, but it didn't. I haven't had a chance to verify, though I could.

Note I also add a couple classifiers.

refs #31

[godber]

I should really probably add to the README before you accept this, but I wanted to get something up sooner rather than later.

[cmbruns]

@godber Thank you for this.

Numpy and PyOpenGL are not strictly required to make use of the openvr wrapper. I suppose they are a bit more required than glfw and all, for the samples. I should probably eventually separate the opengl samples from the core openvr wrapper into a separate module or modules.

[godber]

You're welcome.

Ah yeah, it didn't occur to me that those weren't actually required ... just for the examples. I could add a openvr[all] that does everything or move the numpy and PyOpenGL to be installed with each windowing toolkit. Let me know if you'd like something else.