Rewrite of 6.3 Concurrency

This finalizes the rewrite of chapter 6, together with the previous
commit 4fb3ca2.
Should have been in a new PR, but didn't know how.
small typo in statement table
Merge pull request #248 from creighton/stmttbltypo

Small typo in Statement table
Merge pull request #247 from bscSCORM/concurrency

Rewrite of 6.3 Concurrency
Merge pull request #8 from adlnet/master

updating