refactor basic tile
date and subjects fields are read only and must be obtained from the object itself at rendering time
in fact fields can not be RO because we need to configure them
fix is_empty method and add some tests for it
add TODO message to populate_with_object method
on Archetypes we have no issues, but on Dexterity we could have content types that do not have the IReferenceable behavior enabled
refactor render tests
remove comment as this functionality was already implemented
use use date_format_long on template
remove needless default message for field
restore date and subjects fields to the tile persistent data
enhance module docstrings
New implementation of Subject rendering & fix test suite.
rename method that return the categories of the original object, à la Dublin Core
add a couple of categories on the test (closes #109)
Merge pull request #111 from collective/109

refactor basic tile