Update Plugin.GenServer to match the same pattern that Plugin.Supervisor does, remove Omnibot.Plugin

* using Plugin.GenServer now uses Plugin.Base so that is not required
* Remove Omnibot.Plugin because all it did was include Plugin.GenServer
  and Plugin.Base

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/plugin/gen_server.ex

@@ -1,7 +1,7 @@
 defmodule Omnibot.Plugin.GenServer do
   defmacro __using__([]) do
     quote do
-      alias Omnibot.Plugin
+      use Omnibot.Plugin.Base
       use GenServer 
 
       def start_link(opts) do