Add application/jose+json and header value support

[michaelrsweet]

ACME and other protocols use the JWS JSON Serialization format (application/jose+json) in addition to plain JSON. In addition, some values (such as the "kid" parameter) appear in the protected header that cupsJWTSign and cupsJWTHasValidSignature currently don't support.

Tasks:

• [x] Add "kid" key/value pair when signing with cupsJWTSign
• [x] Lookup "kid" value when verifying with cupsJWTHasValidSignature
• [x] Add cupsJWTSetHeaderNumber, cupsJWTSetHeaderString, and cupsJWTSetHeaderValue to set values in the JWS Protected Header (no support for unprotected values)
• [x] Update cupsJWTExportString and cupsJWTImportString to support the JWS JSON Serialization

[michaelrsweet]

[master 8ff320898] Support exporting to JSON serialization (Issue #52)

[master 1f7b9b0c2] Select/match keys (Issue #52)

[master 03e5a9c0d] Import JSON serialization formats (Issue #52)

[master 3b0471d7a] Initial work on adding JSON serialization support (Issue #52)