Add `Environment` tab to SparkUI.

This adds a tab which displays system property and classpath information. This
can be useful in debugging various types of issues such as:

1. Extra/incorrect Hadoop jars being included in the classpath
2. Spark launching with a different JRE version than intended
3. Spark system properties not being set to intended values
4. User added jars that conflict with Spark jars
Responding to Matei's review
Merge pull request #699 from pwendell/ui-env

Add `Environment` tab to SparkUI.