MRG: doc hyperlinks, fixed size thumbnails

This PR enhances the examples in the documentation by

- Embedding hyperlinks to the example code on the website
- Use fixed size thumbnails to improve alignment

This is still WIP; the documentation links for matplotlib are currently broken (due to recent changes on the matplotlib website). Also, the thumbnail generation code requires PIL, let me know if this dependency is a problem. I could try to do the same using matplotlib.

This PR should work well together with #1581 where it was first discussed. The hyperlink code is from mne-tools/mne-python. Have a look here to see the result:

http://martinos.org/mne/auto_examples/index.html