Allow specifying amounts in drops (#892)

* Accept "drops" in lieu of "XRP"
* Export xrpToDrops() and dropsToXrp()
* Throw our own validation errors instead of BigNumber Errors

docs/src/prepareOrder.md.ejs

@@ -22,6 +22,8 @@ All "prepare*" methods have the same return type.
 
 ```javascript
 const address = 'r9cZA1mLK5R5Am25ArfXFmqgNwjZgnfk59';
+
+// Buy 10.10 USD (of the specified issuer) for 2.0 XRP (2000000 drops), fill or kill.
 const order = <%- importFile('test/fixtures/requests/prepare-order.json') %>;
 return api.prepareOrder(address, order)
   .then(prepared => {/* ... */});