[SPARK-3454] Expose JSON representation of data shown in WebU

This is the result of JSON for environment.

    {
    "Runtime Information" : {
      "Java Home" : "/usr/java/jdk1.7.0_65/jre",
      "Java Version" : "1.7.0_65 (Oracle Corporation)",
      "Scala Version" : "version 2.10.4"
    },
    "Spark Properties" : {
      "spark.serializer" : "org.apache.spark.serializer.KryoSerializer",
       ...    
      "spark.tachyonStore.folderName" : "spark-a8ac134b-187d-4d4b-b9ef-140e626ee96c"
    },
    "System Properties" : {
      "java.io.tmpdir" : "/tmp",
      ...
      "java.version" : "1.7.0_65",
      "sun.io.unicode.encoding" : "UnicodeLittle"
    },
    "Classpath Entries" : {
      "/home/sarutak/work/spark/sql/hive/target/scala-2.10/test-classes" : "System Classpath",
       ...
      "/home/sarutak/work/spark/assembly/target/scala-2.10/spark-assembly-1.1.0-SNAPSHOT-hadoop2.2.0.jar" : "System Classpath"
    }
  }