Round 2: Updated IPython rendering

Replaces #80. Decided on a new pull request to clean up the commit history.

Changes:

 * `imolecule.min.js` uses version in openbabel/contributed - no build changes to openbabel.
 * On first run, attempts to grab `imolecule.min.js` from internet and install in `~/.ipython/nbextensions`. Enables subsequent local usage if successful, falls back to remote library otherwise.
 * Notebooks now statically render in e.g. nbviewer ([example](http://nbviewer.ipython.org/github/patrickfuller/nbviewer-test/blob/master/openbabel_test.ipynb)).
 * Uses IPython's require.js to significantly lower memory usage for multiple molecules in one notebook.