[SPARK-12833][SQL] Initial import of spark-csv

CSV is the most common data format in the "small data" world. It is often the first format people want to try when they see Spark on a single node. Having to rely on a 3rd party component for this leads to poor user experience for new users. This PR merges the popular spark-csv data source package (https://github.com/databricks/spark-csv) with SparkSQL.

This is a first PR to bring the functionality to spark 2.0 master. We will complete items outlines in the design document (see JIRA attachment) in follow up pull requests.

Author: Hossein <hossein@databricks.com>
Author: Reynold Xin <rxin@databricks.com>

Closes #10766 from rxin/csv.