Use namespaces

In relation to ticket to [drop the WPCOM prefix](https://github.com/Automattic/liveblog/issues/71), all of the files could be namespaced, and therefore class names simplified even more.

`Automattic\Liveblog` would seem like the most obvious namespace to use.

Start by reviewing ticket #71 and the repository's PHP class layout to determine the full scope of the proposed namespace change. Since no files or tests are named, inventory the affected classes and define completion as consistently applying the Automattic\Liveblog namespace while preserving existing behavior.

Written by the indexing model from the issue text.