Add etags for POST as a SHOULD

Addresses https://github.com/adlnet/xAPI-Spec/issues/464
Requirement to use anchors in IRIs; minor tidy ups.
Fix example from last commit
Moves IRI definition to requirements; adds new definition.
Make explicit LRS SHOULD re-query when more link is accessed
Clarify anonymous groups
make annoymous groups use even clearer
fixes to POST stags following discussion
Changes to PR #524 agreed on yesterday's call
Changes to PR #531 as discussed on yesterday's call
'more link' -> 'more property'
Removes added groups requirements disapproved of on yesterday's call.
Merge pull request #520 from garemoko/etags_for_POST

Etags for post
Merge pull request #524 from garemoko/issue-270

Metadata retrieval language covering the case of anchor URLs
Merge pull request #531 from garemoko/issue-336

Make explicit LRS SHOULD re-query when more link is accessed
Merge pull request #532 from garemoko/issue-353

Agents in anonymous group
Clarify canonical language process
Fix typo
Further clarifying revisions of IRI definition and requirements
Merge pull request #527 from garemoko/issue-470

Tidy up IRIs definition