Skip exceptions

This PR implements a way to skip over exceptions when running codecells in a notebook, as discussed in https://github.com/ipython/ipython/pull/4993 by @astrofrog. This is a rebase of the two core files, adds documentation and a JS test.

The Javascript code to run all cells in a notebook while skipping exceptions then looks like this:

```Javascript
for (var i=0; i < IPython.notebook.ncells(); i++) {
    IPython.notebook.select(i);
    var cell = IPython.notebook.get_selected_cell();
    cell.execute(skip_exceptions=true);
    }
```

Thanks, sounds reasonable.