Support dynamic filenames.  Fixes IBMStreams/streamsx.hdfs #4.

This uses the second idea described in IBMStreams/streamsx.hdfs #4--namely, the second attribute on the input stream is the filename.  

Some notes:
* removed the requirement for the file parameter 
* At compile time, check that:
    * If dynamicFilename is unspecfied, then file must be specified.
    * If dynamicFilename is unspecified, then there must be one attribute on the input stream
* At runtime, if dynamicFilename is unspecfied or false, then there must be one attribute on the input stream, and file must be specified.

Also added a sample application in HDFSFileSinkExamples.