Add tests to Trello plugin

[ifosch]

Features to implement:

• TrelloPlugin creation
• TrelloPlugin.organization on different cases
• TrelloPlugin.load_orgs()
• TrelloPlugin.load_boards()
• TrelloPlugin.orgs
• TrelloPlugin.boards
• TrelloPlugin.org_names
• TrelloPlugin.board_names
• TrelloPlugin.get_board(board_name)
• TrelloPlugin.list_orgs()
• TrelloPlugin.list_boards()
• TrelloPlugin.list_column_cards(column)
• TrelloPlugin.list_board_columns(board_name)
• TrelloPlugin.list_objects(bot, update, args)

Fakes/spies to implement:

• TrelloClient
• os.getenv