Partial update to v2 API.

- Update request urls and headers
- Update models User, Company, etc.
  - Update field name 'custom_data' to 'custom_attributes'
  - Update User social_profiles field
  - Remove User field last_impression_at
  - Add User field 'remote_created_at'
  - Update User url
  - Update User companies
  - Add companyId field
- Use httpPost for updateUser, and add custom serializer for Companies
- Modify some tests