Support for additional flavors of redmine/custom attributes

[impoetk]

Would it be possible to allow customization of the fields of the issue? to be compatible with easy_redmine for example, they have added additional fields like "easy_email_to" to the response, and currently with this library is impossible to customize this or get them, getting a list of additional attributes to get or set would also be enough

[zapadi]

A new serialization model that will be more extensible is in works