Bad full sync performance

[yuuki0xff]

NOTE: This issue will be fixed after #153 is fixed.

During the full syncing, nvpy sends many requests. But these sends one request at a time. The round trip time between Tokyo and Simplenote API server is about 140ms ~ 150ms. In this way, depending on the geographical location with the server, the time required for sync will increase.

To mitigate this effect, nvpy should support the multiple connections or websocket API.