Updated ObjLoader with group loading functionality.

I cleaned up the ObjLoader and added group loading #553.
It's now possible to load by index or name.
I made a change to the constructor to indicate whether or not to parse normals and tex coords.

### Use cases
Load everything:
```c++
ObjLoader loader( loadFile( filePath ) );
auto batch = gl::Batch::create( loader, gl::getStockShader( gl::ShaderDef().color() ) );
```
Load group named Object:
```c++
ObjLoader loader( loadFile( filePath ) );
auto batch = gl::Batch::create( loader.groupName( "Object" ), gl::getStockShader( gl::ShaderDef().color() ) );
```
Load group index 1:
```c++
ObjLoader loader( loadFile( filePath ) );
auto batch = gl::Batch::create( loader.groupIndex( 1 ), gl::getStockShader( gl::ShaderDef().color() ) );
```