Add internal :camelize_params option to List

Since *most* of the YouTube API "list" endpoints require camelCase keys,
an internal parameter called `:camelize_params` has been added to the List
action which allows to reduce duplication of code, and to indifferently
write Ruby parameters either in underscore or camel-case form.

The API "list" endpoints which *do not* require camelCase keys have the value
set to `false`; the default value is `true`.