Conversion fixes

Addressed some invalid 0.9 statements
Added conversion for non-URI extension keys
Fixed typo
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.
modify context activities, including description tweaks and array values. Fixes #28. Related to #75, which might be okay to close.
shorten long line lengths
Moved inverse functional identifiers into their own sub-section

This change started as an attempt to clarify that actors do NOT require inverse functional identifiers, rather they require a valid agent or group -- which in turn require inverse functional identifiers except for anonymous groups. In order to help clarify that I gave inverse functional identifiers their own sub-section, since talking about IFI's being required in the actor section makes it sound like they're required for all actors.
minor changes based on feedback

* fixed a typo
* re-worded definition of Inverse Functional Identifier
Merge pull request #147 from bscSCORM/actor

Moved inverse functional identifiers into their own sub-section
rephrase singleton to single
Fixed small typo pointed out by @bscSCORM
Merge pull request #145 from fugu13/arraycontextactivities

Changes to contextActivities as discussed in #28
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
Merge pull request #139 from bscSCORM/conversion

Conversion fixes -- ALSO REVIEW: all of Appendix E