get rid of AtomicCommand interface
everything is documentable, so get rid of the Documentable interface
convert get_full_synopsis() method to a standalone function
Simplify dispatch class hierarchy:

* merge AbstractCommandContainer into CompositeCommand
* make RootCommand inherit from CompositeCommand
* get rid of redundant Command and CommandContainer interfaces
make Subcommand inherit from CompositeCommand
get rid of pre_invoke() method
move find_command() utility to Help_Command class

it's not used anywhere else, nor should it be
Merge pull request #508 from wp-cli/dispatcher-refactor

Refactor dispather classes