update accordion widget
update headline widget
update image widget
update google maps widget
bugfix: rename image widget example migration class
update person widget
update slider widget
update slideshare widget
update teaser widget
update text widget
update video widget
update vimeo widget
update youtube widget
update column generator

+ Split into two separate and more explicit generators that create a
column2 and a column3 widget.
+ It is no longer possible to create a generic column widget by
specifying the number of columns.

fixes #277
remove Api::WidgetGenerator

+ adopts the widget generator, which now only creates the placeholder
files which the developer has to fill out
don’t use attribute titles anymore
remove widget USAGE files as they are not used
remove unused method
remove double module inclusion
rename model generator to obj generator

+ remove command line options to set attributes and other things that
can be edited directly in the generated files