ENH: Add basic helpful debug info

This at least will get us some useful info from `vispy.sys_info()`. Let me know if there are other good things to add, or if that's not a great method name. Current output on my Ubuntu system:
```
Platform: Linux-3.11.0-15-generic-x86_64-with-Ubuntu-13.10-saucy
Python:   2.7.5+ (default, Sep 19 2013, 13:48:49)  [GCC 4.8.1]
Backend:  PyQt4 (qt)
Qt:       PyQt4 /usr/lib/python2.7/dist-packages/PyQt4/QtOpenGL.so
Pyglet:   pyglet 1.2alpha1
glfw:     glfw (3, 1, 0)
glut:     from OpenGL 3.0.1
```