Add configuration for double opt-in, defaults to false
Add a warning in case double-opt-in is disabled while subscribe-by-default is enabled
Allow double_opt_in boolean to be passed to initializer of the list.

Default to true to keep specs passing for now.
Pass the double_opt_in config option to the initializer of the list
Fix specs so the default parameter value is no longer needed
Update initializer comments & read me
Change segment api to accept an array of emails instead of a single email, like the mail chimp API itself.
Add rake task for segmenting users

Rake task will segment all subscribed users and print out errors if it encounters any.
Add more detailed status output. Display exactly how many segments succeeded successfully.
Add spec for segmenting
Add another spec to check if newly created users get segmented
Update read me
Merge branch 'feature/double-opt-in' of https://github.com/johanb/spree-hominid

Conflicts:
	app/models/spree/chimpy/configuration.rb
	lib/spree/chimpy/interface/list.rb
	lib/spree_chimpy.rb
	spec/lib/chimpy_spec.rb
	spec/lib/list_interface_spec.rb