Add support for non absolute sibling directories

This PR covers [issue #198](https://github.com/yeoman/grunt-usemin/issues/198) where a file structure looks like this:

```
+ app
  + html
    + index.html
  + scripts ...
    + bar.js
    + baz.js
```

and index.html has this usemin block:
```
<!-- build:js ../scripts/foo.js -->
<script src="../scripts/bar.js"></script>
<script src="../scripts/baz.js"></script>
<!-- endbuild -->
```

I'm trimming the '../' when the block is parsed and adding it back in after the revved candidates are found.
Added some test coverage and updated the readme.