Add content item classes

[michaelwheeler]

These would act as a replacement for pylti1p3.deep_link_resource.DeepLinkResource. It seems like the only crucial interface is an as_dict method.

Defining content item classes within django-lti would allow for an improved API (specifying properties on init). It would also open the door for broader support of standard content item types and properties, which has been a frequently desired improvement to pylti1p3.

• Deep Link ltiResourceLink response type should support iframe property
• deep link: support HTML content type
• Add html embed option for links and html option for Deep Link Resource Class