Added spec for libsass issue 713

[KittyGiraudel]

I am not sure what I should write in expected_output.css since the expected result is a Sass error. I'll edit as needed.

[xzyfer]

This is blocked on #136.

[KittyGiraudel]

Alright.

[xzyfer]

@hugogiraudel since each of the three function results in an error can you please create a tests for each with the appropriate error message in the expected_output.scss

spec/libsass-todo-issues/issue_713/
|- and
  |- input.scss
  |- expected_output.scss
|- or
  |- input.scss
  |- expected_output.scss
|- not
  |- input.scss
  |- expected_output.scss

[KittyGiraudel]

Appropriate error message according to the Ruby compiler?

[KittyGiraudel]

Done.

[xzyfer]

Thanks!