introduce WP_CLI\Loggers namespace
add back WP_CLI::out(), just to prevent fatal errors
Loggers\Regular: don't expose $handle parameter
keep error_to_string() calls inside WP_CLI

They assume WP is loaded, by calling is_wp_error().
don't use is_wp_error() in error_to_string()

We want to be able to use it without WP being installed.
Merge pull request #470 from wp-cli/loggers

Move logging functionality to separate classes