Add ReadTheDocs compatibility

This pull request would close https://github.com/spinus/sphinxcontrib-fancybox/issues/2 by adding compatibility with ReadTheDocs.org.

Three things were changed:
+ fancybox was upgraded to v2.1.5, which is required for the version of jQuery that ships with Sphinx 1.2+ and with ReadTheDocs
+ The ``copy_stylesheet`` function was connected to ``'builder-inited'`` because ``'build-finished'`` didn't seem to be triggered on RTD.
+ A check was added to the logic in ``copy_stylesheet`` to ensure stylesheets are copied on RTD.

If you want to see it in action, take a look at: http://image-analysis.readthedocs.org/en/explicit-fancybox/06_photometry_intro/photometry_aij.html#setting-up-aperture-photometry and click on the image next to item 3, subitem 5 (sorry that example is so obscure--I really like this functionality and wanted to integrate it asap)