Remove the Apache Commons-Lang dependency in scalding-date

This is a fix for issue #643. It replaces the call to the DateUtils.truncate() method with one to a new CalendarOps class that reimplements truncate in less code. I copied the unit tests from the original class to insure that the new method functions as a drop-in replacement. I also removed the jar dependency from the build file and another unused import that referenced the commons-lang library from scalding-core.