Show user-defined job name in UI

This patch displays the "spark.job.description" property in the web UI, to let users or higher level applications (such as Shark) annotate stages with job-level information.

It also does some code clean-up (`addLocalProperties` --> `addLocalProperty`).

![image](https://f.cloud.github.com/assets/320616/904481/93b93ba8-fbc6-11e2-951c-12422d1c28ec.png)