restructure and update the contrib examples

[jensheit]

Split example directory into toy examples and task specific examples.
Update the example training scripts so that the experiments use similar directory structures.
Add evaluation scripts if missing and update the evaluation directory structure, so that the evaluation files are written in a new directory inside the training directory.

• create io.py and add function for default training directory definition @boeddeker
• move to toy examples and update mnist, multi_gpu, configurable @boeddeker
• remove the acoustic model example . @boeddeker
• rename and update the simple train script for audio tagging, remove the advanced training script @JanekEbb
• add evaluation script for audio tagging @JanekEbb
• move, rename and update the training script for mask estimator @jensheit
• add evaluation script for mask estimator @jensheit
• move, rename and update the or_pit and tasnet example @thequilo @jensheit
• move and update the pit example @TCord
• update the speaker classification example @michael-kuhlmann @JanekEbb
• add evaluation script for speaker classification @michael-kuhlmann
• move and update the wavenet example @JanekEbb