Implement getAccessToken
Added optional data parameter to getAuthorizationRequestUrl,
getAccessToken and getRefreshToken methods
Refactored to make use of Zend\Http\Client instead of
\Zend\Http\ClientStatic
Added invalid access token endpoint url handler
Added configuration subkey constants
Added ZendService\Oauth2\Http\Exception classes
Removed error_log lines
Formatted source code
Corrected signature of ZendHttpClient constructor signature