Rename cache directory

[aimeos]

Currently, the package creates a cache directory but it would be better to use a hidden directory named as the package like .potrans or (even better if you may want to add a configuration file option later) .potrans/cache. Are you open for a pull request changing the directory name? If yes, do we need a migration strategy for the existing directory?

[OzzyCzech]

sure, cache directory will be much better in .potrans/cache. Migration strategy is not necessary, cache just save requests to server