Added boolean option :full_error_messages to bootstrap_form_for

Allow the form to not render the div containing full error messages.
As indicated in the new test, if you write the following, you won't get the error_messages div.

    <%= bootstrap_form_for @project, full_error_messages: false do |f| %>
      <%= f.text_field :name %>
      <%= f.submit %>
    <% end %>

The normal behaviour hasn't been altered.