Previously, a set of standard signals would be handled automatically via
an init() function, however that made the package difficult to use in
packages in which an HTTP server would only be spawned some of the times
(perhaps keyed on an environment variable or flag). Now, signals must be
registered manually.

By default, the top-level "goji" package automatically registers
signals with graceful, so this will result in no behavior changes for
most people.

Fixes #35.

graceful.Server was made private in 05c2ca7e, but I think the increased
flexibility you get with being able to provide your own TLS options
(etc.) outweighs the API complexity of an additional type.

For sendfile(2) support. I should really DRY up the implementations here...

Apparently stdlib support will be in Go 1.3. Yay!

Package graceful provides graceful shutdown support for net/http servers,
net.Listeners and net.Conns. It does this through terrible, terrible hacks, but
"oh well!"