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.

db/migrate/20111214091851_create_web_hooks.rb

@@ -0,0 +1,9 @@
+class CreateWebHooks < ActiveRecord::Migration
+  def change
+    create_table :web_hooks do |t|
+      t.string :url
+      t.integer :project_id
+      t.timestamps
+    end
+  end
+end