Fixing the date randomizer, adding test for checking it

First of all, I like your project and I can use it in my work. However I started writing small unit tests to test the project and DataRangeRandomizer did not work, it would always generate date out of specified range.

I have fixed this and added test for it. Please consider to merge it.

Moreover, I would advise to use joda time (http://www.joda.org/joda-time/) in implementation (rather than java Date). Of course, leave java date in the interface for the users. I would like to contribute to moving from java date to joda time if you make that decision.