don't store token in module attribute, fix dep ref

Module attributes are evaluated at compile time. If a user builds the Nostrum dependency before setting up their config.exs file, they'll run into issues with the :nostrum config options. Additionally, if a user wants to change their token they would also have to recompile the app. There's no reason to store it in the module attribute, so lets get it directly from the app env to prevent all of this.

Author: Kraigie

README.md

@@ -21,7 +21,7 @@ end
  Stable documentation can be found [here](https://hexdocs.pm/nostrum/)
 ```elixir
 def deps do
-  [{:nostrum, "~> 0.1"]
+  [{:nostrum, "~> 0.1"}]
 end
 ```
 

docs/static/Intro.md

@@ -65,7 +65,7 @@ end
  *Stable*
 ```Elixir
 def deps do
-  [{:nostrum, "~> 1.0"]
+  [{:nostrum, "~> 1.0"}]
 end
 ```
 

lib/nostrum/api/base.ex

@@ -2,7 +2,6 @@ defmodule Nostrum.Api.Base do
   @moduledoc false
 
   @version Nostrum.Mixfile.project[:version]
-  @token Application.get_env(:nostrum, :token)
 
   use HTTPoison.Base
 
@@ -23,7 +22,7 @@ defmodule Nostrum.Api.Base do
 
   defp process_request_headers(headers) do
     user_agent = [{"User-Agent", "DiscordBot (https://github.com/kraigie/nostrum, #{@version})"} | headers]
-    auth = [{"Authorization", "Bot #{@token}"} | user_agent]
+    auth = [{"Authorization", "Bot #{Application.get_env(:nostrum, :token)}"} | user_agent]
     auth
   end