Refactor and simplify Finder#where
Minor spec cleanup (unnecessary assertions)
Allow symbols as role names in Task::Finder query
Add OR grouping and child finders to Task::Finder

This now allows us to join queries with an "or" operation, and also to
generate complex queries (grouped "or" operations ANDed with ungrouped
queries).  Also fix #available which didn't previously work in #where,
but was not caught by tests.
Fix #available for "or" finders, and add inverse

The #available query needed to change to explicitly be an "and" join
between unclaimed and completable.  Now that "or" joins are possible,
the inverse of #available is also possible - tasks that are claimed
OR not completable.
Test for unrecognized query type on Task::Finder

Though it's not currently possible to directly add to the @queries array
on a Finder, it's important that we don't ignore an unrecognized type,
as it could cause a false negative.
Spelling error in spec name
Add Finder#unavailable as inverse of available