Changed ci::ObjLoader to enable loading of groups from .obj files.

This is somewhat of a quick fix in response to #553 but it enables the following use case:
```
map<string, gl::BatchRef> mGroups;

ObjLoader loader( loadAsset( "model.obj") );
int index = 0;	
for( auto group : loader.getGroups() ) {
	loader.load( index++ );
	mGroups.insert( pair<string, gl::BatchRef>(group.mName, gl::Batch::create( loader, gl::getStockShader( gl::ShaderDef().color() ) ) ) );
}

mGroups["groupName"]->draw();
```
For the general case of loading the full obj the following must be called:
```
loader.load();
```
Before creating the Batch:
```
gl::Batch::create( loader, gl::getStockShader( gl::ShaderDef().color() ) )
```
Therefore the ObjLoader sample must also be updated.