Add support for non absolute sibling directories

[danemacaulay]

This PR covers issue #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.

[danemacaulay]

close in favor of #412