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 
Indeed, a string like a|b|c would be exploded into a and b|c. So this seems to be a non-issue actually.