remove note stating RSASSA-PSS not implemented
add Crypto.JOSE prelude module
rename JWS.Header to JWS.JWSHeader

Since the prelude module will export the JWS and JWE modules, once
JWE is implemented, it is necessary to distinguish between JWS
Headers and JWE Headers.  Therefore rename "Header" to "JWSHeader".
Consequently, the JWE Header type will be named "JWEHeader".