Move the en-GB language plugin into the core

We need to have the default language baked into the core so that we
don’t have problems if no language plugins are available.
Fixed problem with field mauling when reading skinny tiddlers

The problem was showing up as tiddlers with the field “fields” and the
value “[Object object]”.
Docs updates
Sync from server even when not logged in
Specify encoding for incoming requests

Fixes #397