Need a multiclass example (1 example multiple classes)

#### Issue Description
This is a fairly common use case:
Sigmoid output
Xent output

An iterator that knows how to map a set of files/rows to multiple labels.

Please describe your issue, along with:
- expected behavior
- encountered behavior

#### Version Information

Please indicate relevant versions, including, if relevant:

* Deeplearning4j version
* platform information (OS, etc)
* CUDA version, if used
* NVIDIA driver version, if in use

#### Contributing

If you'd like to help us fix the issue by contributing some code, but would
like guidance or help in doing so, please mention it!