Merge pull request #5 from adlnet/master

update from ADL master
All properties on context are optional

Also added some missing </td> tags.
Change yes/no to Required/Optional for attachments
section 4.2 required/optional on tables
section 5.4 required/optional on tables
section 7 required/optional columns on tables
Merge pull request #434 from garemoko/requiredcols_contextonwards

Add optional/required for the last set of tables