Refactor Sword::Task interface (especially class methods)

We don't mimic Thor.desc and Thor.long_desc anymore, that's just
ridiculous. Instead we use RubyGems-inspired more general attributes:
usage, summary and description.

Usage looks like 'task <required_arg> [optional_arg]'
Summary looks like 'Prints welcome message'
Description is a more detailed version of summary, usually in 30-40 lines