Write complete and detailed test code for Date

This task consists on reviewing QuantLib/C++ Date against our Date interface, implementing the missing methods and writing additional test cases in order to make sure that the API is expressive and intuitive enough. Some background regarding this task is as follows.

I detected that interface Date had some methods missing. In particular, some methods involving Period were missing and methods for adding and subtracting dates were missing. This fact leaded to side effects on other classes which make use of Date.

A good example is: we'd like to calculate a date which is 3 years in the future from the current business day. The code I found was:

Settings s = Configuration.getSystemConfiguration(null).getGlobalSettings();
Date today = s.getEvaluationDate();
Date newDate = today.increment(3 * Period.ONE_YEAR_FORWARD.getLength());

... which seems to be OK at first glimpse but in fact, is copiously wrong for these reasons:

1. getEvaluationDate does not guarantee we have a business day in our hands;
2. getLength() returns the length of a period. In this case it's 1, i.e. 1 year. It means we are incrementing today by 3, i.e: 3 days.
3. increment changes the calling object. It means that todays 'is not today anymore' :(

The correct code would be:

Calendar calendar = Target.getCalendar();
Date today = calendar.advance(DateFactory.getFactory().getTodaysDate());
Date newToday = today.plus(Period.ONE_YEAR_FORWARD.times(3));

I implemented times, plus and minus. I added the option of passing a Period to these and some other methods.

More test cases are needed in order to exercise the API and detect situations like the shown above, where something 'seems' to be OK but is the other way round.

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Relationships
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has duplicate http://bugs.launchpad.net/bugs/jquantlib-396
related to http://bugs.launchpad.net/bugs/jquantlib-380
child of http://bugs.launchpad.net/bugs/jquantlib-327