Add contact page and its test

- Add contact page
- Add test
Revise the content of contact page

- Revise the content of contact page to be the same as the tutorial.
Refactor the test for static_pages

- Refactor the redundunt test code for static_page by using "let" directive.
Introduce postgresql

- install pgsql with homebrew
- apply the patch to avoid the preinstalled pgsql startup
- configure environment variables on your .bashrc
- create a user on pgsql
- edit database.yml to connect pgsql
- run bundle exec rake db:migrate