Make event handlers optional to define

Check if event handlers have been defined before attempting to call them.
This prevents the bot from crashing in the case that an event handler
hasn't been defined.

lib/twitter_ebooks/bot.rb

@@ -70,13 +70,13 @@ module Ebooks
       @stream.on_event(:follow) do |event|
         next if event[:source][:screen_name] == @username
         log "Followed by #{event[:source][:screen_name]}"
-        @on_follow.call(event[:source])
+        @on_follow.call(event[:source]) if @on_follow
       end
 
       @stream.on_direct_message do |dm|
         next if dm[:sender][:screen_name] == @username # Don't reply to self
         log "DM from @#{dm[:sender][:screen_name]}: #{dm[:text]}"
-        @on_message.call(dm)
+        @on_message.call(dm) if @on_message
       end
 
       @stream.userstream do |ev|
@@ -110,9 +110,9 @@ module Ebooks
         # - Or soft-retweeted by somebody else
         if mentions.map(&:downcase).include?(@username.downcase) && !ev[:retweeted_status] && !ev[:text].start_with?('RT ')
           log "Mention from @#{ev[:user][:screen_name]}: #{ev[:text]}"
-          @on_mention.call(ev, meta)
+          @on_mention.call(ev, meta) if @on_mention
         else
-          @on_timeline.call(ev, meta)
+          @on_timeline.call(ev, meta) if @on_timeline
         end
       end
     end