refactored the api to return complex content.
Content() and Entry() methods now return interface{}, which can potentially be any IB content type
It is up to the caller to check the type before making use of the data (e.g. switch t := response.(type) )
the point is that we need to support collections of collections being returned by the /content endpoint. this is a pain in the ass.
the Article() and Video() endpoints were left alone, since they are commonly used and very convenient
ability to parse image and gallery types
added Image, Gallery, and related subtypes to model and created methods to parse same in responses
also added additional testing for these methods, content methods
moved all test fixtures into their own file, and started using testify asserts for tests
created helper method to set up testing servers for each test using httptest package
added convenience method to retrieve Image object, this wraps content API call similar to Article and Video methods
added util method to extract displayable media (i.e. articles, videos, images, and galleries) from potentially nested collections