[WIP] SPARK-2450: Add YARN executor log links to UI executors page

This adds a new column in the Executors page of the Spark UI called "Logs", visible only when running under YARN. After getting a container for an executor, the YarnAllocationHandler informs the UI of where its logs reside, so that a link can be presented in the Logs column for it.

Current issues / limitations:
* This only works running with yarn-cluster, not yarn-client.
* The mechanism for getting the log URL through to the UI is different than the usual way information gets there (via SparkListeners). There may be a better, more decoupled way to do it.

![screen shot 2014-07-10 at 4 36 28 pm](https://cloud.githubusercontent.com/assets/1541206/3557128/006f6ea2-092c-11e4-8125-5c2c3a158068.png)