Added game/list endpoint for fetching filterable lists of games

Added a `game/list` endpoint to fetch a list of games using filters by game property. Currently the only supported filter is by `status`, which will be used to implement [galaxy/15](https://github.com/cvan/galaxy/issues/15). This endpoint is implemented in an abstract fashion, however, so adding additional filters in the future should be trivial. Filters can also be restricted to users with certain permissions, so use of the `status` filter, for example, is restricted to reviewers and admins. There is also primitive support for limiting results by `count`. 

The implementation for fetching and filtering games will need to be refactored at some point for scalability reasons, as we currently have to fetch the entire list of games from the DB in order to filter them by property. We'll likely also have to support paginated fetching of games from the DB, which would factor in to how `count` is currently used. For now, though, this implementation works.

Fixes #87.