On Tuesday, PayPal hosted its first developer conference in San Francisco to officially open its platform to software developers who want to include payments in their Web or cellphone applications. With the new platform, any software developer can relatively simply embed PayPal’s technology in an application. An application could be as simple as ordering and then paying for a pizza with one click on a cellphone. Or it could be as intricate as a way for corporate accounting departments to pay vendors without mailing checks.