Added jQuery library

The jQuery library gives us a great range of utilities for easier
handling of documents.
Some of the functionalities are duplicated in D3, but some are not. In
a real world project we would reconsider if jQuery is strictly
necessary, but for making this class as easy as possible we will just
include it.
Added .gitignore

A .gitignore file inside your Git repository will prevent the git code
versioning from seeing and capturing files, that you don't want inside
the repository.
For instance: if you save passwords or stuff like that in configuration
files, you do not want these files to appear in a public Github account
- therefore you should add sensitive files to the .gitignore.
Other files to be hidden are those typical OSX artefacts like .DS_Store
or project metadata files from your favorite texteditor.
Moved the Javascript code into an external file.

We prefer to write our javascript code in separate files, because it
makes it easier to debug them using developer tools in the browser.