Support for bintray organizations

Right now the bintray task assumes that the bintray subject and user are the same entity. This is not the case with organizations. So this patch introduces an an explicitly delineated user parameter for the BASIC_AUTH settings. This allows a user to specify an organization as a subject, which will be different to the user. To maintain compatibility with the existing behavior, what I have I done is to make the user parameter optional. When it is set explicitly in the task config, it will be used. If it is not set, then the subject parameter will be used. So by default, the existing behavior is preserved. But if you need to specify an organization, you can.