Add an Audio display class

This is a first attempt at addressing issue 4241.
Fix unnecerary dependence to numpy
Move imports into methods and fix docstring typo
Fix talk about image embeding in the Audio class.

Too much copy paste I guess :)
Change to encodestring to b64encode to avoid newlines in the data URI

The point of this is to improve compatibility with older browsers.
also, encodestring is depricated in python 3.
Fix extra ) syntax error
Fix broken normalizing when signal array already has integer type

A case of unwanted integer division.
Speed up normalization if numpy is avaliable
Make it explicit that we don't support encoding stereo PCM signals
Remove support for encoding audio without numpy for performance
Add Audio display to the Rich display example notebook
Remove requirement for numpy module
Make examples in docstring  produce a less annoying sound.
Run the Audio cells in the Part 5 example notebook
Merge pull request #4302 from davidosterberg/master

Add an Audio display class