support notebooks in %run

Simple enough - just generalized safe_exec_ipy a bit to support a sequence of cells rather than just one cell (the .ipy behavior).

I think we have an issue open for this, but I can't seem to find it.
make %run magic complete on .ipynb files
test %run notebook.ipynb
add %run notebook.ipynb to what's new
Merge pull request #4452 from minrk/nbrun

support notebooks in %run