Use submit_tag in example of submit button overwrite

The content_tag helper could be used to generate a <button>, but it is
better to use the input[type="submit"] tag.
Use ERB as template engine of form html file

This will remove the dependency of haml.
Uniformize use of Hash syntax

Since the new (from Ruby 1.9.3) it's already used in specs and some lib
files, use it abroad the project.