nbconvert: Export flavors & PDF writer (ipy dev meeting)

- Lightweight subclasses reduced in quantity
- Added post processors
- `--format` param is now `--to`
- `--post PDF` flag added which creates and uses the new (introduced in this PR) `PDFPostProcessor`

        See the following excerpt from help:

        You can specify the export format with `--to`.
        Options include {0}
        
        > ipython nbconvert --to latex mynotebook.ipnynb

        Both HTML and LaTeX support multiple output templates. LaTeX includes
        'basic', 'book', and 'article'.  HTML includes 'basic' and 'full'.  You 
        can specify the flavor of the format used.

        > ipython nbconvert --to html --template reveal mynotebook.ipnynb
        
        You can also pipe the output to stdout, rather than a file
        
        > ipython nbconvert mynotebook.ipynb --stdout

        A post-processor can be used to compile a PDF

        > ipython nbconvert mynotebook.ipynb --to latex --post PDF
        

See requirements listed in dev meeting hackpad:
https://hackpad.com/IPython-Summer-2013-Development-Meeting-D1UR23usGnA