priority queue no longer extends a standard queue, no advantages to that
Merge branch 'master' into options
save a function call by checking if index is not null
renamed priority_map to priority_table to avoid confusion, updated tests
refactored priority list logic into abstract class and implemented priority list, and priority map classes. updated priority queue tests, added new tests for list and map.
append() should not rely on set(), too confusing
massively simplified index validation on insert/delete
Merge branch 'master' into options
priority must be an integer, added public priority update method and tests
Merge branch 'master' into options
implemented priority map structure to replace ad hoc hierarchy