Add ipython[notebook] requirements.

#### What's this PR do?
Adds in the ipython notebook requirements.

#### Where should the reviewer start?
The only files changed are `requirements.txt` and `.gitignore`.

#### How should this be manually tested?
Run `pip install -r requirements.txt` on a clean machine.  This does not include the install of zeromq though, which would need to be handled on a per system/OS case.

#### Any background context you want to provide?
I wanted to checkout these ipython notebooks for examples of data analysis with pandas, but noticed that it only installed pandas and assumes ipython[notebook] is installed system-wide (or was just left out of the `requirements.txt` file).

Feel free to disregard this PR if you don't think it is necessary.  I just wanted to help others that found this repo to get up and running with the least amount of resistance. :cake: