Add OCP\Security\IHasher

Public interface for hashing which also works with legacy ownCloud hashes and supports updating the legacy hash via a passed reference.

Follow-up of https://github.com/owncloud/core/pull/10219#issuecomment-61624662
Requires https://github.com/owncloud/3rdparty/pull/136

@DeepDiver1975 
It is already at https://github.com/owncloud/core/pull/11947/files#diff-6f2ab7037b9c3db62415e7b6ac8567e8R18

Also `|` can also be inside an hash as it is exploded into only two substrings. - Any idea how to document this better?