Withdrawal script (#4)

* Withdrawal script. Added a script that lets you withdraw through Kraken API if the charged fees are under a certain level

* refactored code and added validation functionality

* Enhanced README to reflect the option to also withdraw in an automated manner

* fixed typo

package.json

@@ -8,6 +8,8 @@
   "main": "index.js",
   "scripts": {
     "stack-sats": "node index.js",
+    "withdraw-sats": "node withdraw.js",
+    "test-withdraw-sats": "node withdraw.js --validate",
     "test": "node index.js --validate"
   },
   "dependencies": {