Add a hover menu to the "Agents" nav link.

I found myself tired of performing a click on "Agents", scroll down and then one more click every time I create a new agent or view the diagram, so I decided to add a hover dropdown menu to the "Agents" nav link.

![agents_hover_menu](https://cloud.githubusercontent.com/assets/10236/3889645/5dcd9822-2212-11e4-81ce-afeb656f87e9.png)

What do you think?  Should it require a click for the menu to open?  In that case, the "Agents" nav link becomes a menu opener and an item "Your Agents" should be added to the menu.