GPG key post: Add link for web-of-trust

Link to the GnuPG manual section covering it.
GPG Anatomy Post: Change 'Master' to 'Primary'

Per DKGs recommendation.
Anatomy post: structure->certificate

Also add a definition and link for trust, validity, and web of trust.
Anatomy post: Limits of the short key id

Add a link to DKG's post on the "bad idea".