Curly class methods appear private, but aren't

https://github.com/zendesk/curly-templates/blob/master/lib/curly.rb#L49-L64

The class methods `compile_reference` and `extract_references` are defined after a `private` declaration, but that declaration doesn't apply to class methods. If you meant those to be private, it should be

```ruby
class << self
  private

  def compile_reference(reference)
    ...
  end

  def extract_references(template)
    ...
  end
end
```