Add handling for "content" elements that link to the content rather than embed it

In some Atom feed formats, like Visual Studio private galleries, "content" is used to link to the actual content rather than embedding it. You can see that [in Microsoft's example](http://msdn.microsoft.com/en-us/library/hh266717.aspx), where content is an empty-element tag with a "src" attribute pointing to the VSIX package file.

I've added handling for this case like so:
  - If "self.content" is a dictionary object, or a dictionary-subclass object, add every key & value as attribute name & value, respectively. However, if one of these is named "content", treat it as actual content and embed it between a start-tag and end-tag (the default behaviour).
  - If "self.content" is any other type, treat it as you treated it before.

This should help in special cases like above without breaking existing scripts.