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/models/web_hook.rb

@@ -0,0 +1,20 @@
+class WebHook < ActiveRecord::Base
+  include HTTParty
+
+  # HTTParty timeout
+  default_timeout 10
+
+  belongs_to :project
+
+  validates :url,
+            presence: true,
+            format: {
+              with: /(^$)|(^(http|https):\/\/[a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,5}(([0-9]{1,5})?\/.*)?$)/ix,
+              message: "should be a valid url" }
+
+  def execute(data)
+    WebHook.post(url, body: data.to_json)
+  rescue
+    # There was a problem calling this web hook, let's forget about it.
+  end
+end