[SPARK-7500] DAG visualization: move cluster labeling to dagre-d3

This fixes the label bleeding issue described in the JIRA and pictured in the screenshots below. I also took the opportunity to move some code to the places that they belong more to. In particular:

(1) Drawing cluster labels is now implemented in my branch of dagre-d3 instead of in Spark
(2) All graph styling is now moved from Scala to JS

Note that these changes are related because our existing mechanism of "tacking on cluster labels" afterwards isn't flexible enough for us to fix issues like this one easily. For the other half of the changes, visit http://github.com/andrewor14/dagre-d3.

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**Before.**
<img src="https://cloud.githubusercontent.com/assets/2133137/7582769/b1423440-f845-11e4-8248-b3446a01bf79.png" width="300px"/>

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**After.**
<img src="https://cloud.githubusercontent.com/assets/2133137/7582742/74891ae6-f845-11e4-96c4-41c7b8aedbdf.png" width="400px"/>

Author: Andrew Or <andrew@databricks.com>

Closes #6076 from andrewor14/dag-viz-bleed and squashes the following commits:

5858d7a [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-bleed
c686dc4 [Andrew Or] Fix tooltip placement
d908c36 [Andrew Or] Add link to dagre-d3 changes (minor)
4a4fb58 [Andrew Or] Fix bleeding + move all styling to JS