Shutter: Specify validator and keyper (#11)

* Shutter: Specify validator and keyper

Co-authored-by: Max Langenfeld <[email protected]>

* Consistently specify language in code blocks

* Fix syntax errors in code snippets

* Store (de)registration messages in contract storage

Nodes often don't have access to historical events, so in order to get
the list of registered validators we now store the registration and
deregistration messages in the contract storage.

* Move definition of get_events function

* Fix typo and wording

---------

Co-authored-by: Max Langenfeld <[email protected]>