Different link rendering

Introduce a 'start to start' method to render dependencies, retaining the start-to-end behaviour as the default.

At the moment, this won't be possible to call without your own version of ganttDrawer - there's no modifications to the dependency parsing to set a link type.

However, it does refactor the rendering code slightly; and in later pull requests the wiring up can be added.

![image](https://f.cloud.github.com/assets/365751/166223/fa65d454-7991-11e2-8960-09041a93a6b9.png)