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
switched upconversion prefix from urn: to tag: as decided on call
Merge pull request #172 from bscSCORM/conversion

switched upconversion prefix from urn: to tag: as decided on call
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
add section on data constraints
attempt to deal with spacing issue
more space fiddling
minor wording change
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
Merge pull request #176 from fugu13/dataconstraints

Add a section on data constraints