Fix example generation

Issue #222 reports the following error when building the docs:

    OSError: [Errno 2] No such file or directory: '/Users/me/src/vispy/doc/examples/techniques/grids'

I fixed that with a single-line change. `os.makedirs` is the recursive version of `os.mkdir`. the `grids` directory was getting created, but `techniques` didn't exist.

However, I am now getting the following error:

    Exception occurred:
      File "/home/david/build/vispy/doc/ext/glapigenerator.py", line 36, in main
        glext_const_names, glext_func_names = parse_api(gl.ext)
    AttributeError: 'module' object has no attribute 'ext'

Also, the following works in my shell:

```python
In [5]: from vispy.gloo import gl

In [6]: gl.ext
Out[9]: <module 'vispy.gloo.gl.ext' from '/home/david/Dropbox/.virtualenvs/frackoptima/lib/python3.4/site-packages/vispy/gloo/gl/ext.py'>
```

So it seems to me `gl.ext` is generated somewhere when importing in the shell, but not in Sphinx.