Add years, months and weeks

[nene]

Years are really hard. As a scientific unit:

1 year == 365.25 days

It's hard to deal with fractions.

Weeks are seemingly simple.

1 week == 7 days

But when you try to express a year in weeks, things get ugly:

1 year == 52.1785714285714... weeks

Weeks are now implemented in weeks branch.

When we are going to introduce years, we better introduce months too, because no-one can understand something like this:

2 years and 24 weeks

But the following is a whole lot meaningful:

2 years and 6 months