Etags for post

Addresses https://github.com/adlnet/xAPI-Spec/issues/464

This one will need some discussion and may be related to the other discussions around methods raised by @canweriotnow and @ryansmith94

Note that 
* I have added POST to the descriptive text in all cases.
* I have added the requirements relating to specific headers as SHOULDs in the spirit of not adding new MUST requirements
* I have left the requirement around how to handle failed preconditions as a MUST, but looking at this again it should probably be SHOULD. I'll change this following discussion. 
* I've not added anything around the requirement for an LRS to reject in the case of no etag headers being present as even a MAY here would imply a MUST for activity providers. 

This is not ready to be merged until more fully discussed.