Decode OIDs from ASN.1 BER for display

The bytes in the gss_OID_desc structure are the ASN.1 BER encoded form of the OID. When displaying the OID we should show the human-readable form of the OID. In my python-gssapi library I added a dependency on pyasn1 just for decoding OIDs, but in the interest of keeping dependencies simple, I wrote a basic decoder for the BER encoded OID data. This makes the `repr()` of OID objects match the human-readable form:

before:

```python
>>> MechType.kerberos
<OID [42, 134, 72, 134, 247, 18, 1, 2, 2]>
```

after:

```python
>>> MechType.kerberos
<OID [1, 2, 840, 113554, 1, 2, 2]>
```