Querying voided statements
Statement query changes

clarified that the multipart attachment format is to be used in responses when attachments are requested even if there are no attachments.
simplified existing query options while expanding them. (see extensive changes)
group filter updates, activity & group equality tweaks
targeted statement search clarificaton
Moved single-statement queries in to the main query parameter block in order to allow "attachments" and "format" to apply
added "voidedStatementId" to allow fetching voided statements
added "viewId" to allow limiting statement results on any pre-defined views the LRS provides
changes based on feedback

* returned SHOULD to MUST for including members in ad-hoc groups
* removed "viewId" query parameter
changes based on feedback
* made related_activities and related_agents behavior more specific
Based on feedback, reverted most group changes
* reverted all changes to group requirements to before this PR
* aside from impact on related_agent flag, and some "how to match on groups" clarifications, removed group query changes except to forbid anonymous groups as a filter option for agent parameter

Leaving prohibition of anonymous groups as a query parameter (and specificatoin that for an identified group specified
as a search parameter only the identifier is relevant) to avoid requiring LRS to search on a list of agents.
change based on feedback

* changed case of "subStatement" to "SubStatement" to be consistant
changes based on feedback
* added link to voided statements from voidedStatementId parameter
* added more detail to category description
missed that we already had a "voided" anchor
Formatting fix
minor fixes based on feedback

* removed apparently accidental suggestion that stored could be used in substatements
* added link to sub-statement query behavior in related voided requirement
* typo fix
removed some redundant language re: querying groups
Merge pull request #96 from bscSCORM/query

Major query overhaul