Changed url link in Help dropdown menu.

Trying to resolve #4057. 

I removed "iPython Help" link from the drop-down help menu and replaced it with "Notebook Examples"
that links to http://nbviewer.ipython.org/github/ipython/ipython/tree/1.x/examples/notebooks/.

Let me know if you have feedback on this new url change or if you have alternative ideas to style the help menu.