Small style changes
Move diff failure reporting to assuming success instead of failure.

Enables runs to transition to the failed state when the pdiff process fails for some reason, while still reporting the pdiff log for human inspection.
Fixed bug where sending a task heartbeat would not invalidate the build cache.
Enables filtering a work queue list by task status.

Links to these views from the work queue index page.
Evict the build cache when a task related to a build is finished.

This ensures that the final status of the task is up-to-date in template rendering. Without this if a task finishes after having failed once before, the failure status will continue to show on the run page.
Reworking the readme for clarity.