moving abstract classes for Entry in another module to avoid circular importation in a normal shell
fix tests and kwalitee
rename EntryAbstractClass to AbstractEntry
update attr to ignore when building the API
remove specific code for Python2.5 in the tests
Merge branch 'master' into fix-entry-import-219

Conflicts:
	zinnia/tests/entry.py
update documentation about extending the entry model, close #219