ftp: port is not mandatory. Closes #2.

* src/dctc/java/com/dataiku/dctc/GlobalConstants.java: Constants for
  ftp port (21).
* src/dctc/java/com/dataiku/dctc/file/FTPFile.java,
  src/dctc/java/com/dataiku/dctc/file/FTPFileBuilder.java: Adjust, and
  port is not mandatory.