#30: changed HDFSUtils.readLines(...) to return a list of Strings - one for each line: better representation of a file which could contain duplicate lines, deduplication is not always required

Made getFilesCount independent of data source (takes in List<String>): better testable.

Removed HDFSUtils.writeProfessionCounts(...): following Pito's golden rule "don't optimize until you have to". getProfessionCounts(...) works in under 3 seconds with the original profession_test.txt. No need to write the counts into a file and read it in later.
#30: refactored the tests. Added getProfessionCounts(...) performance test.
The performance test shows that reading in the whole profession_train.txt with over 673k article -> professions assignments as well as parsing and counting - all in total - takes less than 5 seconds. Thus we don't need methods to write and read the counts into / from a separate file.
#30: added JUnit test for HDFSUtils.getProfessionCounts(...).
Closes #30.