Ripple calc

* New uint160 tagged type NodeID.
* Categorize *all* non-generic instances of uint160 as Account, Currency or NodeID.
* Move conversions to and from strings to central .h file.
* Extract macro constants to be functions.
* Get rid of constructors and comparators between tagged uint types.
* New base_uint::copyFrom() allows explicit copying between any uint types
  of the same size.
* And lots of incidental cleaning and removal of unused code.
Well, I picked the best name I could, given the code.

We have two special accounts:

uint160(0) - XRP issuer - all clear on that one!

uint160(1) - guaranteed not to be a "real" account, so it's used as a placeholder when you're wanting to represent "No account".

Take a look at the usage of ACCOUNT_ONE and you'll see I mean - I think!