(maint) http/convert-uuids only converts necessary
(maint) Refacter group-resource decision chain.

Create explicit predicates for the kinds of requests group-resource can
service, and use those to make decisions. This should be more
understandable than the previous interwoven checks on request method and
whether something was retrieved.
(maint) Remove argument-schema? from middleware.

It wasn't actually detecting argument schemas, since those can have
names that aren't "argN". It can be replaced with just a `sequential?`
check.
(NC-162) Create groups via POST.

Add the capability to create a group by POSTing to the groups collection
endpoint. This is done by reusing the group-resource function, but
passing it nil instead of the UUID from the URI (since there is none).

Since it can now be passed nil, group-resource needs to check if the
uuid argument its given is nil before using it in a few places.

Update the `http/malformed-group?` docstring to describe the validation
it does.
(NC-162) Update docs for POST group creation.
Merge pull request #53 from aperiodic/nc-162-group-post

(NC-162) Create groups with posts.