Find component file relative to config file

RE: https://github.com/yeoman/yeoman/issues/1078

It seems like the component file is being looked up relative to the directory where the `grunt bower` command is run. So when it sees it has to include a file `bower_components/jquery/jquery.js`, it finds it relative to the directory the command has been run in. In the case of a `yo webapp`, that's one directory up from app, so that was resulting in `app/bower_components/jquery/jquery.js` being injected into main.js. I switched it to use the config file's directory as the "working directory", and it seems to be working well :+1:

I had to add a `../` to the tests' paths to account for this.