Wallet: use dustThreshold directly

The definition of a dust amount is pretty clear, and I feel it is less
readable when represented as isDust(amount) or !isDust(amount), by
comparison to    amount <= dustThreshold   or   amount > dustThreshold.

Also means I don't have to stray my eyes to understand the
implemention by looking up isDust does.

src/wallet.js

@@ -22,6 +22,9 @@ function Wallet(seed, options) {
   this.addresses = []
   this.changeAddresses = []
 
+  // Dust value
+  this.dustThreshold = 5430
+
   // Transaction output data
   this.outputs = {}
 
@@ -168,7 +171,7 @@ function Wallet(seed, options) {
   }
 
   this.createTx = function(to, value, fixedFee, changeAddress) {
-    if (isDust(value)) throw new Error("Value must be above dust threshold")
+    if (value <= this.dustThreshold) throw new Error("Value must be above dust threshold")
 
     var utxos = getCandidateOutputs(value)
     var accum = 0
@@ -189,7 +192,7 @@ function Wallet(seed, options) {
       if (accum >= subTotal) {
         var change = accum - subTotal
 
-        if (!isDust(change)) {
+        if (change > this.dustThreshold) {
           tx.addOutput(changeAddress || getChangeAddress(), change)
         }
 
@@ -205,12 +208,6 @@ function Wallet(seed, options) {
     return tx
   }
 
-
-  this.dustThreshold = 5430
-  function isDust(amount) {
-    return amount <= me.dustThreshold
-  }
-
   function getCandidateOutputs(value){
     var unspent = []
     for (var key in me.outputs){