Allow fetching the image from an alternate tag

- and in the Sheldon source, set that to "content".
Allow fetching namespaced tag content

- Added routine for fetching content tag; will fetch from namespace if
  a tag namespace is present.
- Updated Sheldon source to define a namespace.
Fixes to namespacing

- s/latest/item/ in getContent() method
- actual name of Sheldon tag is "encoded"
Merge branch 'hotfix/sheldon-fixes'

Fixed sheldon comic fetching properly