eventual consistancy

* Created a section for common requirements when PUT-ing or POST-ing statements,
moved existing common requirements to that section
* Added eventual consistancy requirements
tidy up
Merge pull request #171 from bscSCORM/eventual

eventual consistancy
Merge pull request #177 from fugu13/tidyup

Just a small bit of tidying up
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
Merge branch 'master' into dataconstraints
Rearrange LRS requirements related to API parameters for clarity and applicability
fix typo in header name