Pr ipython directive context

```
Running Sphinx v1.1.3
loading pickled environment... not yet created
loading intersphinx inventory from http://statsmodels.sourceforge.net/devel/objects.inv...
loading intersphinx inventory from http://docs.python.org/objects.inv...
building [html]: targets for 2 source files that are out of date
updating environment: 2 added, 0 changed, 0 removed
reading sources... [ 50%] dsintro                                                                       

>>>-------------------------------------------------------------------------
Exception in /home/user1/src/pandas/doc/source/dsintro.rst at line 33: # <<<< This is new
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-4-05c9758a9c21> in <module>()
----> 1 1/0

ZeroDivisionError: integer division or modulo by zero
<<<-------------------------------------------------------------------------

```

No more excruciating "guess where" on errors.