Added web hooks functionality

This commit includes:

 * Projects can have zero or more WebHooks.
 * The PostReceive job will ask a project to execute any web hooks defined for that project.
 * WebHook has a URL, we post Github-compatible JSON to that URL.
 * Failure to execute a WebHook will be silently ignored.

app/workers/post_receive.rb

@@ -1,5 +1,8 @@
 class PostReceive
   def self.perform(reponame, oldrev, newrev, ref)
-    puts "[#{reponame}] #{oldrev} => #{newrev} (#{ref})"
+    project = Project.find_by_path(reponame)
+    return false if project.nil?
+
+    project.execute_web_hooks(oldrev, newrev, ref)
   end
 end