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.

spec/spec_helper.rb

@@ -8,6 +8,7 @@ require 'rspec/rails'
 require 'capybara/rails'
 require 'capybara/rspec'
 require 'capybara/dsl'
+require 'webmock/rspec'
 require 'factories'
 require 'monkeypatch'
 
@@ -48,6 +49,8 @@ RSpec.configure do |config|
     end
 
     DatabaseCleaner.start
+
+    WebMock.disable_net_connect!(allow_localhost: true)
   end
 
   config.after do