Feature: Feature requests from Issues + Simple Client

[kelvin-lu]

Feature requests from issues:

• return a promise from logEvent that returns the promise from when the event is sent. (Issue #12 )
• create a simpler client that does not use retry or flush, goes straight to transport layer
  • on this client, a logEvents(...events) WIP (issue #23 )

[kelvin-lu]

Closing this in favor of another PR: https://github.com/amplitude/Amplitude-Node/pull/30