Support for Groups API

This release adds support for Mixpanel Groups, through the mixpanel.groups object, analogous to mixpanel.people. For projects which have groups support enabled, you can send group profile updates via calls such as:

mixpanel.groups.set('company', 'Acme Inc.', {
    'Industry': 'widgets',
    '$name': 'Acme Inc.',
});

mixpanel.groups.remove('company', 'Acme Inc.', {
  'products': 'anvil',
  'customer segments': 'coyotes'
});

mixpanel.groups.delete_group('company', 'Acme Inc.');

// etc