[CHEF-4062] Intro type option to cookbook create

http://tickets.opscode.com/browse/CHEF-4062

knife cookbook create now supports the option --type=TYPE or
-t TYPE for specifying the 'style' of cookbook to create. The
default behavior remains unchanged (type 'full'). Additionally
the 'minimal' type is defined which creates the following
cookbook structure and file(s):

    recipes/default.rb
    metadata.rb
    README                      (of whatever format)
    CHANGELOG.md

Some refactoring was required to sanely support the new feature
and some other seemed obvious to do. For instance, default recipe
priming was being done in create_cookbook yet there are all sorts
of other create_NNN methods for readme, changelog, etc. We now have
create_cookbook_tree and then create_default_recipe IFF the tree
was made with a recipes directory. Some future cookbook types might
not want a recipes directory, and that's okay with this code.