Merge nbconvert into IPython

Supersedes PR #3497.  This PR merges the actual repo, and is made from the IPython org so all core devs can finish it up.

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`?