Jinja Templates

Re-write Nbconvert using Jinja Template. 

Mostly a draft branch. 

It Is **much more easier** to tweek and to make inheriting template, 
without having to code a class that inherit from Converter.py

look at `converter/converter_template.py`
and 
`templates/*.tpl`

I considere `templates/python.tpl` almost done, and which fixes some 
strangeness of previous implementation that had inconsistencies in blank lines. 

usage 

`./runme.py template name`

generating a diff with current ipynb references: 

```
diff -U3  tests/ipynbref/IntroNumPy.orig.html <(./runme.py fullhtml)
```

I would really like the though of @ellisonbg, @fperez.
Pinging @damianavila  which might be interested.