Update xAPI.md

grammar and wording clarifications in section 2
Update xAPI.md

grammar and wording improvements in section 1
Update xAPI.md

grammar and wording in section 3
Update xAPI.md

grammar and wording in section 4.0 and 4.1 intro.
Fixed small typo pointed out by @bscSCORM
Merge remote branch 'upstream/master' into mike
fixed merge conflicts
Conflicts:
	xAPI.md
Merge pull request #151 from bscSCORM/mike

Mike's cleanup PR with merge conflicts fixed