Fix considering extended directory using .bowerrc

Rebased and cleaned up version of #942. I can really see this is considered a bug.



```
cat .bowerrc
{
    "directory": "app/assets/components"
}
bower install angular-bootstrap
```

Before: 

```
bower list -p
{
  "angular-bootstrap": "components/angular-bootstrap/ui-bootstrap-tpls.js",
  "angular": "components/angular/angular.js"
}
bower list -p --relative=false
{
  "angular-bootstrap": "/Users/sheerun/Source/bower/app/assets/components/angular-bootstrap/ui-bootstrap-tpls.js",
  "angular": "/Users/sheerun/Source/bower/app/assets/components/angular/angular.js"
}
```

After:

```
bower list -p
{
  "angular-bootstrap": "app/assets/components/angular-bootstrap/ui-bootstrap-tpls.js",
  "angular": "app/assets/components/angular/angular.js"
}
bower list -p --relative=false
{
  "angular-bootstrap": "/Users/sheerun/Source/bower/app/assets/components/angular-bootstrap/ui-bootstrap-tpls.js",
  "angular": "/Users/sheerun/Source/bower/app/assets/components/angular/angular.js"
}
```

After fix it behaves the same if `config.directory` is not specified.