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

src/api.ts

@@ -1,5 +1,5 @@
 import {EventEmitter} from 'events'
-import {Connection, errors, validate} from './common'
+import {Connection, errors, validate, xrpToDrops, dropsToXrp} from './common'
 import {
   connect,
   disconnect,
@@ -300,6 +300,9 @@ class RippleAPI extends EventEmitter {
   signPaymentChannelClaim = signPaymentChannelClaim
   verifyPaymentChannelClaim = verifyPaymentChannelClaim
   errors = errors
+
+  xrpToDrops = xrpToDrops
+  dropsToXrp = dropsToXrp
 }
 
 export {