base option not working as expected

Thanks for this plugin, it's been very useful to me.

My understanding is that the `base` option provides a mechanism to output assets to the given directory. Perhaps it's just that I'm misunderstanding how the base option works.

I've created a test repos that hopefully demonstrates the problem: https://github.com/MozMorris/gulp-useref-example

I set the base option like so:

``` javascript
.pipe(useref({
searchPath: 'static',
base: 'dist/some/other/directory'
}))
```

HTML:

``` html

```

My expectation is that assets in my `static` folder get written to `dist/some/other/directory` with the final html looking like:

``` html

```

The actual result is my assets are written to `dist` without the base option folder structure.