add Project::createMilestone()

[cmbirk]

Implementing the createMilestone as mentioned in the documentation.

As a heads up, and differing from the documentation, the Teamwork API requires responsible-party-ids, so $args should look like:

[
        'title'                 => 'Some Title',
        'description'           => 'Some Description',
        'deadline'              => '20150402',
        'notify'                => false,
        'reminder'              => false,
        'responsible-party-ids' => '1234567'
]