internationalization

Changes in this PR

1. Setup locale according to http accept language

2. installed a gem 'http_accept_language' to help extract and compare accept/ available locales 

3. specs
Please break it up to two lines:
```ruby
client_language = http_accept_language.compatible_language_from(I18n.available_locales)
I18n.locale = client_language || I18n.default_locale
```