This allows users to signal Goji to exit ungracefully by means of
sending two signals in quick succession.

This backends to the listener.DrainAll functionality I just added,
allowing impatient users to shut down their services immediately and
un-gracefully if they want.

Exactly the same code, just (hopefully) organized a little more
sensibly.

This change refactors package graceful into two packages: one very well
tested package that deals with graceful shutdown of arbitrary
net.Listeners in the abstract, and one less-well-tested package that
works with the nitty-gritty details of net/http and signal handling.

This is a breaking API change for advanced users of package graceful:
the WrapConn function no longer exists. This shouldn't affect most users
or use cases.

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.

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!"