make full_html default export format
cleanup flags / aliases for NbConvertApp

follow conventions elsewhere in IPython
RST class capitalization
populate exporter list in NbConvertApp.classes
s/ConfigurableTransformer/Transformer/
populate NbConvertApp.classes from exporters, writers, transformers
export_format is an enum

and options are included in help output

examples expanded
use positional args *or* config

not both
mention local config file in examples
Merge pull request #1 from minrk/nbconvert

Cleanup on NbConvertApp