Aggregate the number of bytes spilled into disks during aggregation or sorting, show them in Web UI.

![spilled](https://cloud.githubusercontent.com/assets/40902/4209758/4b995562-386d-11e4-97c1-8e838ee1d4e3.png)

This patch is blocked by SPARK-3465. (It includes a fix for that).

Author: Davies Liu <davies.liu@gmail.com>

Closes #2336 from davies/metrics and squashes the following commits:

e37df38 [Davies Liu] remove outdated comments
1245eb7 [Davies Liu] remove the temporary fix
ebd2f43 [Davies Liu] Merge branch 'master' into metrics
7e4ad04 [Davies Liu] Merge branch 'master' into metrics
fbe9029 [Davies Liu] show spilled bytes in Python in web ui