Return value in bytes if the attribute is not currently handled + add object dictionary for printing

[Muzosh]

Hey, I would like to propose a fix to an information loss.

Sometimes, the library does not know, how to "unpack" certain attributes. I think it would be great to at least return the value in byte array. Currently, the code just raises an exception while providing just the key, but some developers might utilize the returned value and translate it themselves.

[MatthiasValvekens]

This was addressed in a somewhat different way in #209. Defaulting unhandled attribute types to bytes would effectively mean that adding support for a new attribute type in a future version of python-pkcs11 would result in an API-breaking change.

Instead, #209 allows the user to supply an alternative attribute mapper when opening a session. This alternative attribute mapper can include handling for attributes that are not supported in python-pkcs11. The only limitation--which it shares with the implementation proposed in this PR--is that the handler doesn't have access to the PKCS#11 internals (i.e. you're fine as long as you can implement it in Python). In practice, that's not much of a restriction AFAICT, other than for CKA_XYZ_TEMPLATE attributes (which take a raw pointer) and perhaps some vendor-specific stuff.