[FLINK-1081] Add HDFS file-stream source for streaming

You can see more detailed information for this pull request in [JIRA](https://issues.apache.org/jira/browse/FLINK-1081).

There are three big changes in this pull request.

1. Rename FileStreamFunction class to TextStreamFunction class
2. New FileStreamFunction class with feature watching given path
3. Add readFileStream method in StreamExecutionEnvironment class
@rmetzger Oh, It is good idea. I will implement it.