Specification tweaks

[tplooker]

This PR addresses the following.

• Relaxes the condition in the "Creating a JWM section" that required the JOSE header to feature a "typ" field equal to "JWM"
• Changes the crypto requirement section in the following way
  • Require P-256 as ECC curve for ECDSA, ED25519 curve support is optional.
  • Require P-256 as ECC curve for ECDH-ES, X25519 curve support is optional.
• Bump the recommended character limit for JWM attributes from 8 to 15.

Fixes #38, #40, #41

@troyronda @llorllale

[tplooker]

For now the intention is to merge this PR and open a seperate discussion about what the recommend curves for ECDH and ECDSA should be longer term