Feature / labels

[aaemnnosttv]

This PR fixes #6 with easy handling of PostType labels.

All post types require labels at registration time. Previously, registering a post type with Silk did not set post type labels for all labels which referenced the single or plural forms of the label.

This feature enables the most common use cases to set all custom post type labels using the provided methods:

$post_type->oneIs('Book')
    ->manyAre('Books')
    ->register();

[coveralls]

Changes Unknown when pulling 19d2f5d on feature/labels into * on master*.