Added keybindings for displaying the diagram in the default web browser.

To accelerate my workflow, I found that adding a keybinding to display your diagram in the default web browser would be helpful.

Since yUML is an online service and there is no local compiler, I felt it was justifiable to override the default keybinding for building (super-b) to display the diagram.

Since many developers are used to working with a super-s -> super-b workflow (save and compile), when modelling with yUML, this same workflow can be applied effectively.