support adding AES keytab entries for Active Directory

[rilott]

Microsoft active directory uses it's own AES salt for keytab entries. This PR adds a new function AddActiveDirectoryComputerAccountEntry which handles this, and doesn't modify the normal AddEntry function.

I tried to add this with minimal code changes/duplication.

Partially fixes #416 (computer accounts only)
All tests pass

Thanks!
Ross

[gaxelac0]

The methods worked for me as the salt for the tickets emitted by Active Directory comes with the format
DOMAINsamAccountName

And gokrb5 tries to generate a salt with cname.GetSalt method, concatenating all the Names of the SPN which is wrong, ending with something like this:
DOMAINHTTPservicePrincipalName.DOMAIN

With the salt like that, the decryption will fail.