[SPARK-2705][CORE] Fixed stage description in stage info page

Stage description should be a `String`, but was changed to an `Option[String]` by mistake:

![stage-desc-small](https://cloud.githubusercontent.com/assets/230655/3655611/f6d0b0f6-117b-11e4-83ed-71000dcd5009.png)

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1524 from liancheng/fix-stage-desc and squashes the following commits:

3c69327 [Cheng Lian] Fixed stage description object type in Web UI stage table