Async Crypto Endeavour (#33)

* refactor: make import and creation async - This allows the use of native key generation in the browser

BREAKING CHANGE:

This changes the interface of .create, .createFromPrivKey,
.createFromPubKey, .createFromJSON

src/bin.js

@@ -4,4 +4,10 @@
 
 const PeerId = require('./index.js')
 
-console.log(JSON.stringify(PeerId.create().toJSON(), null, '  '))
+PeerId.create((err, id) => {
+  if (err) {
+    throw err
+  }
+
+  console.log(JSON.stringify(id.toJSON(), null, 2))
+})