Add model examples

[ehmicky]

It is very useful for developers to be able to visualize how a specific model, request body or response body might look instead of just seeing the attribute type.

There are several ways to do this, which might look differently depending on which HTML template we are using (see #182):

• setting schema.example for each model attribute. This approach is probably the best. I think some HTML template might require this to be called x-example instead.
• setting response.examples for each endpoint response.