add create_if_missing to files which will be customized

this makes it more useful to be able to run the generator on
already created cookbooks to update them.
Removes ignoring of CHANGELOG

We now use the CHANGELOG in the cookbook on Supermarket to display it on
the site. Let's remove this in the case that `knife cookbook site share`
obeys chefignore.
Merge pull request #131 from opscode/cwebberOps/fix_chefignore

Removes ignoring of CHANGELOG
Merge pull request #115 from opscode/lcg/create-if-missing

add create_if_missing to files which will be customized
Validate source options for cookbooks