Allow to specify an objectToExportTo for goog.exportSymbol

This allows users to build ol3 without anything exposed in the global namespace. This can e.g. be useful for creating an ol3 AMD module, by simply using a build configuration that contains
```json
{
  "objectToExportTo": "o",
  "compile": {
    "output_wrapper": "define('ol',function(){var o={};%output%return o.ol;});"
  }
}
```