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

- `--format` param is now `--to`
- `--flavor` param added.  Used when template isn't explicitly named.
- `--pdf` flag added which creates and uses the new (introduced in this PR) `PDFWriter`
- Lightweight subclasses reduced in quantity

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 flavors of output. LaTeX includes
        'basic', 'book', and 'article'.  HTML includes 'basic', 'full', and 
        'reveal'.  You can specify the flavor of the format used.

        > ipython nbconvert --to html --flavor reveal mynotebook.ipnynb