add initial test case
make get_command_file() a method of RootCommand

It's only used there and it's very unlikely to be needed elsewhere.
add show_help_early()
add more usages to help.feature
fix E_STRICT error in CommandWithDBObject::create() signature
s/existant/existent/
add test for unknown command
test getting help for a third-party command
Merge pull request #312 from wp-cli/independent-help

`wp help` shouldn't require a WP install