Destination `dest` parameter fails due to incorrect relative stylesheet path lookup

[matmar10]

Steps to Reproduce

Create a directory structure such as:

• css/
  • main.css
• styleguide/
• index.js

var path = require('path');
var livingcss = require('livingcss');

// all of these fail:
livingcss('css/buttons.css', 'styleguide');
livingcss(path.join(__dirname, 'css/buttons.css'), 'styleguide');
livingcss(path.join(__dirname, 'css/buttons.css'), path.join(__dirname, 'styleguide'));
livingcss('css/buttons.css', path.join(__dirname, 'styleguide'));

Actual Behavior

Error: ENOENT: no such file or directory, open '../css/buttons.css'

Expected Behavior

• Styleguide generated in specified directory
• Stylesheets linked (if flag enabled)

[matmar10]

Reproduced by PR #80