don't pass the message through colorization; only the label

fixes #744
remove $label parameter from logging methods

The label text and color are just a way to signal to the user the type of message.

Changing it can lead to confusion. Example:

    WP_CLI::error( 'Something bad happened!', 'Success' );
add unit test for regular logger
fix usage of $label parameter in WP_CLI::error()
Merge pull request #745 from wp-cli/log-colorization

Log methods cleanup