Self-update, download only, and other minor optimisations

• #24 - Don't use db:dump from magerun, always use non-exec version within put command.
• #19 - Add drop table if exists around all exports
• #17 - Add a parameter to get command to download without importing
• #16 - ls command now returns list of available names if no name presented
• #14 - Add self-update command
• #9 - Reduce phar size by using box-project for building phar instead of phar-composer