Add dirname to Converter

It is useful for the Sass converter, as it needs to know the path
to the asset file to process directives like @import. Currently
there is no way for a converter to know where the file come from.

Now the convert method can be as follows:

    def convert
      return Sass::Engine.new(@content, load_paths: [@dirname], syntax: :scss).render
    end
Make asset tags accept nested liquid blocks

For example, the following code will work:

    {% css_asset_tag global %}
      {% for path in page.stylesheets %}
        - {{ path }}
      {% endfor %}
    {% endcss_asset_tag %}

I didn't included tests for this, since it seemed hard to write
such tests with the project's current test patterns. I made a small
change to a spec file just to make the current tests not fail.
gitignore the `staging_path` generated when running the specs
update the version constant to "0.1.6", in order to match the latest version released on rubygems.org
update readme: document usage of yamls inline format for lists (i.e. arrays)
Adjusted Sass Engine options
Removed newlines from asset templates

Newlines are not needed here since the Liquid tag will have a newline
after it.
Add a .travis.yml file for builds.