fix buffer import and use rollup for transpiling the package.

rollup.config.js

@@ -0,0 +1,16 @@
+import pkg from './package.json'
+
+export default {
+  input: 'index.js',
+  output: [
+    {
+      name: 'nostrtools',
+      file: pkg.browser,
+      format: 'umd'
+    },
+    {
+      file: pkg.module,
+      format: 'es'
+    }
+  ]
+}