Fixed bug in relative path generation for scripts and styles.

Relative path generation appears broken, and will always generate an unnecessary leading "../" for style and script resources.

For example, I have the following structure:

* project/
  * dist/
    * release/
      * index.html <= generated by grunt-html-build
      * css/
        * main.css <= referenced, relative file
      * js/
        * main.js <= referenced, relative file

The relative path from index.html to main.css should be "css/main.css". Similarly, the relative path to main.js should be "js/main.js". However, grunt-html-build generates the relative paths as "../css/main.css" and "../js/main.js", respectively. 

This happens because when using path.relative(file1, file2), the "file1" parameter also contains the filename. If we change file1 to path.dirname(file1), the correct paths are generated. Here's an example:

```
var path = require('path');
var result = path.relative('./dist/debug/index.html', './dist/debug/js/main.js');
console.log(result); // <= ../js/main.js (INCORRECT)
```

However, if we use dirname instead, we get the correct relative path:

```
var path = require('path');
var result = path.relative(path.dirname('./dist/debug/index.html'), './dist/debug/js/main.js');
console.log(result); // <= js/main.js (CORRECT)
```

If this