Remove 4.3 Validation of Voided

Resolves issue #327
Add etags for POST as a SHOULD

Addresses https://github.com/adlnet/xAPI-Spec/issues/464
POST statements returns an array of statement ids
Requirement to use anchors in IRIs; minor tidy ups.
Fix example from last commit
Clarifies what should be returned when no statements are matched

This is already the behavior of (at least) all major LRS.
Make explicit type of activityId
Make explicit type of agent
Merge pull request #523 from garemoko/stmts-post-returns-array

POST statements returns an array of statement ids
minor tidy up in section 7
Update my org in contributor list
Merge pull request #530 from garemoko/rustici-software

Making things official
Merge pull request #515 from andyjohnson/master

Remove 4.3 Validation of Voided
Make explicit LRS SHOULD re-query when more link is accessed
Clarify anonymous groups
make annoymous groups use even clearer
Language around Not rejecting Voiding Statements

The current version of the spec is silent on the behavior on what an LRS does if a voiding Statement comes in and the target is not available.  As we've discussed, timing issues could cause this delay and an LRS cannot reject solely on this.  This is a SHOULD NOT for this version and will be a MUST NOT for 2.0.
Added SHOULD to MUST update for 2.0
Update xAPI.md
Merge pull request #3 from adlnet/master

Pull in ongoing changes