Various cleanup / minor issues

* 7.1 Error codes needs some "MUST" https://github.com/adlnet/xAPI-Spec/issues/97
    * Tightened the language to "most appropriate" (400 would be appropriate anywhere 403 would be, but 403 should be used when it's "most appropriate").
    * Removed language which implied 400 return codes were optional, or that other return codes could not appear on any resource

* stored shouldn't be in the future, remaining issue was actually the sub-statement reference, removed https://github.com/adlnet/xAPI-Spec/issues/105
* stored/timestamp precision https://github.com/adlnet/xAPI-Spec/issues/83
attachments should specify binary

Issue: https://github.com/adlnet/xAPI-Spec/issues/107

* attachment's MUST specify a transfer encoding of binary
* LRS SHOULD assume binary transfer encoding even if it was (incorrectly) not specified

There is no need to allow other transfer encodings as these messages are to be sent over HTTP which supports binary
Merge pull request #121 from bscSCORM/cleanup

Various cleanup / minor issues
0.95 to 1.0 changes list added
Merge pull request #124 from garemoko/changelog

0.95 to 1.0 changes list added
Merge pull request #122 from bscSCORM/binary

attachments should specify binary
standardized headings in sections 2 and 3
Merge pull request #125 from garemoko/section2&3headings

standardized headings in sections 2 and 3
Change to statement id description

We now say elsewhere that statement ID SHOULD be set by the activity provider
Section 4 headings to verb
Merge pull request #126 from garemoko/section4headings

headings as far as verb (inclusive)
heading changes down to the end of context
Merge pull request #127 from garemoko/objectheadings

heading changes down to the end of context
headings down to end of section 4
Merge pull request #128 from garemoko/headingsTimestamp

more headings down to end of section 4
sections 5 and 6 headings
Merge pull request #129 from garemoko/headings5

sections 5 and 6 headings