SPARK-2159: Add support for stopping SparkContent on exit() repl command

#### Description
A patch that fixes issue [SPARK-2159](https://issues.apache.org/jira/browse/SPARK-2159) by offering the same stop behavior as when using Ctrl+D (EOF) but with the `exit` or `exit()` repl command.

#### Testing
Added test case in `ReplSuite`
super nit: start multi-line comment on the next line.

(Also, not sure what's the Spark convention here; but internally we always use "//" for multi-line comments, so that it's easy to comment out long blocks of code including any comments.)