Simplify get/get_json sequence

Every case of TrelloClient.get is followed immediately by
TrelloClient.get_json, so it is much simpler if .get just returns the
JSON dict.
Add 'show' subcommand for board

Also requires top-level command to have a 'list' subcommand which does
what 'boards' did. And for consistency, change 'boards' to 'board'.
Add ability to search for boards by name instead of ID
Change "card show" to use card name instead of ID