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
Merge pull request #11947 from owncloud/public-hash-api

Add OCP\Security\IHasher