nbconvert rst output lacks needed blank lines

[matthew-brett]

From current master (b5edafe):

ipython nbconvert --to=rst ~/dev_trees/ipython/examples/notebooks/Part\ 1\ -\ Running\ Code.ipynb 

Gives text like this:

Run a code cell using ``Shift-Enter`` or pressing the "Play" button in
the toolbar above:
In[10]:
.. code:: python

    a = 10
In[11]:
.. code:: python

    print(a)

The problem is the lack of line-breaks before and after the prompts; the following produces the output that I think you want:

In[10]:

.. code:: python

    a = 10

In[11]:

.. code:: python

    print(a)

[ivanov]

matthew has a PR incoming that fixes this