Adding /serve_last_updated endpoint.

Keeps track of files by modification time, not size, when computing
metadata. File content can change in place without changing the length
of the file.

Also, added /serve_last_updated endpoint, which returns the info for the
file that was most-recently modified.
Allow querying a specific watcher by basename of the provided directory.
Merge pull request #3 from mattspitz/add-latest

Adding /serve_last_updated endpoint.