Another major change: renamed actual/effective dates

[John Wiegley]

It has long been a point of confusion over what "actual" vs. "effective" dates
are.  For example:

  2012-02-28=2012-03-01 KFC
      Expenses:Food                $10.00
      Assets:Cash

In this example, I always considered the first date as the "actual" date when
I bought the chicken, and the second date as when it become "effective" in my
account.

But others use these dates to mean other things, so I'm now changing these to
PRIMARY and AUXILIARY dates:

  --actual-dates  => --primary-date
  --effective     => --aux-date

Also the value expressions to reach these values are "date" and "aux_date",
respectively.

The old options and value expressions functions are retained for
compatibility, but are deprecated.

John