Introduce a logger

[artem-smotrakov]

I see that the project uses System.out to print out messages, for example:

nist-data-mirror/src/main/java/us/springett/nistdatamirror/NistDataMirror.java

Line 116 in 8f1faec

System.out.println("Downloading files at " + currentDate);

As a result, an application that uses the library doesn't have much control over this messages. It would be better to use a logger. If this improvement sounds good, I can open a pull request that introduces log4j. Please let me know what you think.

[stevespringett]

Yes, that sounds like a good improvement.