Introduce rspec helpers

Creates template and subject methods for rspec.

The template uses a mock unless ActionView is available. It's
recommended to require 'action_view' only if you want to tests methods
that depends on the template (like number_to_currency for example)

The subject method expects that you define a `object` let which will
reflect the attributes the class being presented. It's recommended to
use a stub for test in isolation
Does not require rails to expose extension methods to presenter class

This helps on isolation testing, avoiding the undefined method error
for the class level helpers