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
Update xAPI.md
Consistent quotation
Merge pull request #26 from fugu13/voidinglanguage

Suggest language around voiding
Merge pull request #533 from andyjohnson/master

Language around Not rejecting Voiding Statements