Rename getAliases to getAliasGroups

This is split from #59 to be able to discuss the rename separately. Reasons:
* This doesn't return a collection of aliases (e.g. an `AliasList` object or an array of `Alias` objects or strings). It returns a collection of alias groups.
* The `AliasGroup` class also haves a `getAliases` method. You would end with something like `$fingerprint->getAliases()->getByLanguage( 'en' )->getAliases()` which looks strange.