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
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 branch 'master' of https://github.com/adlnet/xAPI-Spec
clarify statement ref behavior. Closes #152
line length
Merge pull request #139 from bscSCORM/conversion

Conversion fixes -- ALSO REVIEW: all of Appendix E
Clean up 'HTTP Basic Authentication' statement usage
Missing target anchor in par. 6.4.1
Added signed statements section & example