Label reposition in Editor

![RabbitMQSim-labels](https://f.cloud.github.com/assets/1858212/245251/b4f73742-8a7a-11e2-81bc-b5cc26854aa2.png)

This change moves the labels for nodes in the toolbox and on the editing canvas to be at the bottom rather than in the center of the symbol.  This change also applies to edges to render the labels below the edge lines.