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 
Same code as above. You can combine them. Also, you can identify whether it's phpass or sha1 by looking at strlen($hash) first.