Add after_commit hook

Then i can write a plugin that make push commit after save page automatically.

```ruby
# -*- coding: utf-8 -*-
description 'exec git push after save page'

Page.hook :after_commit do
  Olelo.logger.info "push git history after save page"
  git_path = Config[:repository][:git][:path]
  response = `git --git-dir=#{git_path} --bare push origin master 2>&1`
  if $?.success?
    Olelo.logger.info "git history push success"
  else
    Olelo.logger.error "git history push failure, reason: #{response}"
  end
end
```