oid: now only accept Oid or hex, not raw

Every method that takes an oid has changed what it accepts.

Before it was (in both Python 2 and 3):

- An Oid object
- An hex oid, represented as a unicode string
- A raw oid, represented as a bytes string

Now the behaviour is different between Python 2 and 3.

Now in Python 2 we take:

- An Oid object
- An hex oid, represented as a bytes or unicode string

Now in Python 3 we take:

- An Oid object
- An hex oid, represented as a unicode string

We have dropt direct support for raw strings. To use a raw string first
build an Oid object: oid = Oid(raw=raw)

We have also dropt support for short raw oids. The Oid constructor takes
full oids, if passed short oids the behavior is undefined.