Add unitary test for FileUtils and StringUtils classes

[lufe089]

Complete unitary test into com.variamos.common.core.utilities inside the test source code package
Elaborate the javadoc documentation

[nherbaut]

@lufe089 these classes should be removed in favor of standard java 8 classes such as

FileUtils.readDirectoryFromDirectory -> Files.find
FileUtils.readFileFromDirectory -> Files.find
FileUtils.writeFile -> Files.write
FileUtils.writeFile -> Files.write

StringUtils.multiLine -> something in Strings, but We need to look at the code to understand what this method is doing
StringUtils.formatEnumValue -> same