StreamExecutionEnvironment#continuousSource() method should be renamed to source()

The current code in master did not follow the [latest FLIP-27 discussion|https://lists.apache.org/thread.html/r54287e9c9880916560bb97c38962db7b4d326056a7b23a9d0416e6a7%40%3Cdev.flink.apache.org%3E], where we should have a StreamExecutionEnvironment#source() instead of  StreamExecutionEnvironment#continuousSource(). The concern was that the latter would mislead the users to think that the execution is always in streaming mode while it actually depends on the boundedness of the source.