Add EventfulList and EventfulDict trait types.
Address review comments @takluyver
Added on_events convinience method
Use ternary operator for callback registration
kwarg on_events and hardcoded callbacks
s/this/self
Default view should be the base widget view class
Add failing test for issue gh-6282
Cast module name to bytes for Py2 ModuleType constructor

Closes gh-6282
Merge pull request #6286 from takluyver/i6282

Fix issue 6282
Merge pull request #6228 from jdfreder/eventful-list-dict

Add EventfulList and EventfulDict trait types.
Update wrapperkernels.rst

Fixed typo experssions -> expressions. 

This page also needs to mention the command to get started:

```shell
ipython console --kernel echo
```

And needs to point to creating a profile to use the EchoKernel in a notebook.
Merge pull request #6292 from dsblank/master

Update wrapperkernels.rst
Merge pull request #6284 from jasongrout/patch-6

Default view should be the base widget view class
unidirectional link of traitlets
wrong callback
renaming unilink to directional_link and adding dlink alias
adding a simple test for directional_link