Closes #44

Null attributes are attributes with bigint value 0, meaning that the issuer skipped that attribute. Instead of aborting the session, the irmaclient now allows disclosures of null attributes, when the verifier requests it. This allows the irmaclient to prove that a particular attribute was not issued to it, and allows verifiers to request optional attributes alongside other attributes.

fix: irmaclient now updates its credential instances if new attributes were added by the scheme manager

Updated noauth description to specifically state that it will cause server to reject authenticated requests.

Changed UpdateSchemes to run ParseFolder only when new schemes were actually downloaded, based on suggestion by Sietse.

fix: Stopped keyshare code from paniccing when getCommitments failed for reasons other than missing pin-check.

Within a session request, inner conjunctions requesting specific attribute values
used to be serialized to a map. JSON and Go maps are however unordered, and the
order matters: if the requested attributes are received in a different order then
the session will fail. Inner conjunctions using values are now serialized using
the default Go serializer, resulting in a list of requested attributes with their values.