data: Add ability for `IPaginationSource`s to specify how to get settings. The `IPaginationSource` has a new `getSettingAccessor` method that returns how to access settings values on items in the pagination source. A default accessor is also available, which uses standard attribute lookup. Also clean some stuff in `LazyPageConfigData`.

include README.rst
include CHANGELOG.rst
include LICENSE.rst
recursive-include piecrust *.py mime.types
recursive-include piecrust/resources *
recursive-include tests *.py
#global-exclude .DS_Store
#global-exclude Thumbs.db
#global-exclude Desktop.ini
#global-exclude *.swp
#global-exclude *~
#global-exclude *.bak