[Spark 2060][SQL] Querying JSON Datasets with SQL and DSL in Spark SQL

JIRA: https://issues.apache.org/jira/browse/SPARK-2060

Programming guide: http://yhuai.github.io/site/sql-programming-guide.html

Scala doc of SQLContext: http://yhuai.github.io/site/api/scala/index.html#org.apache.spark.sql.SQLContext
How about:

Spark SQL can automatically infer the schema of a JSON dataset and load it as a SchemaRDD.  This conversion can be done using one of two methods:
 - `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
 - `jsonRdd` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.

We should probably also have a brief section on the rules of schema inference as they are non-obvious.

A future TODO:  There is no reason that we need the json objects to be one per line right?  We can know when an object has not ended and could continue reading past a line break, right?