Exposing the hardcoded defaultValue property as a configuration option for Lines

I had the need to create a chart that would point out deviation from a set goal.
I.E. Historical Student attendance goal of 80%.

As you can see in figure 1 when activating the fill:true parameter the fill will take a baseline of 0 (which is a hardcoded value).
Figure 1.
![before](https://f.cloud.github.com/assets/573569/2033238/e482a98a-8919-11e3-959f-cf85be84fefc.png)

In figure 2 I have the chart working as I need it and probably others do. (exposing the property as a configuration value in the Lines properties.
Figure 2.
![after](https://f.cloud.github.com/assets/573569/2033707/66d54770-8920-11e3-8095-64f1fc36d510.png)


Change is very simple please look at the diff.