Fix reconnect, add auth on pool creation

Support pool creation from url including db, username, and password

asyncio_mongo/auth.py

@@ -0,0 +1,17 @@
+import asyncio
+
+
+class Authenticator:
+
+    def authenticate(self, db):
+        raise NotImplementedError()
+
+
+class CredentialsAuthenticator():
+    def __init__(self, username, password):
+        self.username = username
+        self.password = password
+
+    @asyncio.coroutine
+    def authenticate(self, db):
+        yield from db.authenticate(self.username, self.password)