Move load_images into methods mixin
Be clear that DSL holds instance of a builder
No DSL for #part for now
Refactor dsl spec
PageBuilder#writable?
PageBuilder#set
PageBuilder#add_part
PageBuilder: fix, by_title returns a relation, update_attributes
DSL#part calls add_part rather than building immediately
PageBuilder#build_parts delegates to PagePartBuilder#build
PagePartBuilder builds with ref to Refinery::Page

Also be clear about what is a builder and what is a model
accessor PageBuilder#page
remove call to #keep_part for now
PagePartBuilder store part in accessor
Keeping/cleaning parts
Keep parts by title, not by id
Test for case insensitivity of keep_part
#keep_parts explicitly and for defined parts