More sponsorship methods

[driesvints]

Extend the API with even more useful methods:

// Check if driesvints has a sponsor listing...
$client->hasSponsorsListing('driesvints');

// Check if driesvints can sponsor nunomaduro...
$client->canSponsor('driesvints', 'nunomaduro');

// Check if driesvints is privately sponsored by nunomaduro...
$client->isPrivatelySponsoredBy('driesvints', 'nunomaduro');