Options parameter in files.getFiles() is now optional

In spec, options parameter appears to be optional (https://github.com/pkgcloud/pkgcloud/blob/master/docs/providers/rackspace/storage.md#clientgetfilescontainer-options-functionerr-files--), however if null is passed, the method generates "TypeError: Cannot read property 'limit' of null".
Merge branch 'master' into pr/320
Adding null option handles for amazon, azure