Automate validation

Adds automatic validation by running `rake validate`.

Requirements:
- ruby, rake, w3c_validators, psych
- the site to validate uploaded somewhere

- [x] Make `rake validate` take an option to a remote base uri
- [x] Create gemfile to get dependencies more easily
- [x] Extend validate task to validate css files
- [x] Rename `page_arguments` to `request_arguments`