Support crit header parameter

[Vladimir Dzhuvinov]

The critical (crit) header parameter was added in version 09 of the JWS and JWE drafts:

http://tools.ietf.org/html/draft-ietf-jose-json-web-signature-09#section-4.1.10 http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-09#section-4.1.15

Besides adding the appropriate getter/setter methods we need to consider how to handle header criticality at the library level and how to interface that to crypto provider code and app code.

[Vladimir Dzhuvinov]

Added basic crit header support in commit 8c80c34 .

[Vladimir Dzhuvinov]

Current implementation passes "crit" header processing to the app code. If there is need to do more sophisticated checking -> open new ticket.