Merge nbconvert into IPython

This pull request takes the nbconvert package and inserts into the main IPython project as `IPython.nbconvert`.  While `nbconvert` is not done, we need to get it merged ASAP so continue the work.  In addition to the code dump I have made some basic changes:

- [x] Update imports to begin to develop the official API that people will see when importing various things.
- [x] Fixed the installation logic to include nbconvert.
- [x] Created an `nbconvert` sub-command of `ipython` so users can do `ipython nbconvert`.

I will try to get the Travis tests to pass (mostly import issues), but this code does not have tests.  These will be added in a future PR that @jdfreder has already started on.  One question we should figure out before merging:

- [ ] Should nbconvert be a subcommand or its own command?
- [ ] If it is a subcommand, should we call it `nbconvert` or the simpler `convert`?