Respect bower.config.cwd

Bower has a configuration option `cwd` which allows the *bower_components* directory to be located at a different location than the project's root directory. For example, my *.bowerrc* looks like this:

```json
{
    "cwd": "public",
    "directory": "components"
}
```

We need to take that `cwd` setting (which btw defaults to the project's root directory) into account when searching for the files to be concatenated.