add logging for registered handlers
remove stuttery names

gum.GumServer => gum.Server
jekyll.JekyllHandler => jekyll.Handler
remove gorilla mux dependency, simplify Handler interface
switch jekyll handler to use map[string]string
start of static file handler

This handler parses static HTML files for rel="shortlink" and
rel="canonical" links.  If both are found, a redirect is registered for
the pair.  This is a much more flexible alternative to the
jekyll.Handler, since it works with any static authored or generated
site that publishes these links.
refactor logic for handling non-wildcard redirects

This moves the logic for non-wildcard redirects (that is, simple path to
destination mappings) up into the gum.Server, rather than requiring each
handler to handle it directly.  Currently only the jekyll.Handler was
doing this.  Instead, the gum.Handler interface adds a URLs() method
that returns a map[string]string for the gum.Server to handle.

In moving this logic, it means that each gum.Handler does not
necessarily need to be an http.Handler.  In the case of handlers that
just return a mapping of short URLs to destinations, they can just hand
that map to the server and be done.

This has the added benefit for jekyll.Handler specifically in that it no
longer needs a prefix for URLs it handles.  It hands all of its
redirects, regardless of prefix, to the server and they get handled
appropriately.

Normal precedence rules for registering ServeMux handlers applies,
namely that longer patterns win.  Since the simple path to destination
mappings are registered on "/", any handler that is registered for an
explicit path prefix (like RedirectHandler) will take precedence.
list newer style shortlinks before legacy ones
use channel to pass static mappings from handlers

this now opens the door for handlers to provide additional mappings
after startup.  For example, a file system watcher could provide
mappings for newly created or updated files.
add file walker to static handler to parse files
add filesystem watcher for static handler