style: Less space, correct coding style.

* src/core/java/com/dataiku/dip/utils/DKUtils.java: Less space.
* src/dctc/java/com/dataiku/dctc/utils/CharUtils.java: Correct coding
  style.

* src/dctc/java/com/dataiku/dctc/command/cat/SkipFirstLine.java: Less
  empty line.
cat: Don't display the header if the file doesn't exists.

* src/dctc/java/com/dataiku/dctc/command/cat/CatRunner.java: Here.
Refactor and add test.

* tests/cat.test, tests/head.test, tests/tail.test: Here.