[FLINK-1173] Add socket text stream as a data source for the streaming API

Check more detail information in [JIRA](https://issues.apache.org/jira/browse/FLINK-1173).

1. Add SocketTextStreamFunction class for a new stream source
2. Add socketTextStream method into StreamExecutionEnvironment
3. Add unit test for SocketTextStreamFunction class

Tested with `mvn verify` command.