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.