masher: Add a prioritize_updates method
masher: Prioritized thread dispatching (fixes #93)
tests: Ensure that a stable repo containing a security update gets mashed first
tests: Ensure that a testing repo with a security update gets mashed first
tests: Ensure that multiple repos with security updates mash in parallel
masher: Gracefully handle exceptions from Masher.work
masher: Only expire the override if it exists
masher: Fix a minor docstring typo
Merge pull request #95 from fedora-infra/feature/prioritize

Prioritized pushing