Introduce Fog::AWS.compliant_bucket_names reader.

Attempts to access Fog::AWS::COMPLIANT_BUCKET_NAMES from
outside fog (e.g. carrierwave) result in uninialized constant error.

Impetus for this change is that carrierwave defines slightly different regex
(https://github.com/jnicklas/carrierwave/blob/master/lib/carrierwave/storage/fog.rb#L293)
to determine whether to use path or subdomain style bucket access and
it seems better to re-use this constant from fog. However attempts
to access the COMPLIANT_BUCKET_NAMES constant via ::Fog::AWS::COMPLIANT_BUCKET_NAMES fail.

Am I missing something or does this change make sense?