Currency: Add support for complex currencies. (UInt160)

This patch might regress the performance of the Currency class and by extension
the Amount class. Since Amount is on a lot of hot paths in the client we should
make sure this isn't a major problem.

As for compatibility, this patch is a major change, but it should maintain the
public interface very well, which the exception of some strange edge cases (e.g.
Currency.from_json(1337)), which weren't well-defined before anyway. Any code
that accesses _value directly (shame on you!) will need to be fixed. There
aren't any such references in ripple-client or the rippled test suite, so I
think we're looking pretty good.

test/currency-test.js

@@ -19,8 +19,8 @@ describe('Currency', function() {
     });
     it('from_json("XRP").to_json() == "XRP"', function() {
       var r = currency.from_json('XRP');
-      assert.strictEqual(0, r._value);
       assert(r.is_valid());
+      assert(r.is_native());
       assert.strictEqual('XRP', r.to_json());
     });
   });