redis-backed "mentions" timeline as redundant (given notifications)

now return empty hash instead of throwing a 404 error. When following,
merge into timeline will filter statuses

with YAML-defined defaults. Add admin page for editing global settings. Add "site_description"
setting that would show as a paragraph on the frontpage

resources that require a user context vs those that don't (such as public timeline)

/api/v1/statuses/public   -> /api/v1/timelines/public
/api/v1/statuses/home     -> /api/v1/timelines/home
/api/v1/statuses/mentions -> /api/v1/timelines/mentions
/api/v1/statuses/tag/:tag -> /api/v1/timelines/tag/:tag

to the API