Watch tasks by name so we are notified if they are absent

Requires port to latest sbt-rc 1.0-951c90820f296b4eda7eb9759e8d80f148065cd6

Had to create our own Writes for Completion because it's no longer
public in sbt-rc.
Merge pull request #812 from typesafehub/wip/havocp-watch-by-name

Watch tasks by name so we are notified if they are absent