adding RNN with Word Embeddings tutorial

It would be really nice if we could directly reference code snippets from `rnnslu.py`, like PR #43 enabled for the other tutorials.
The syntax would be something like
```rst
.. literalinclude:: ../code/rnnslu.py
  :pyobject: contextwin
```

or, if you add special comments `# before-contextwin` and `# after-contextwin`,
```rst
.. literalinclude:: ../code/rnnslu.py
  :start-after: # start-contextwin
  :end-before: # end-contextwin
```

This will make it easier to maintain.
Btw, for the moment, there are small differences between the two versions, so you may have to merge them.