Also, move it out of Serve so it's possible to override.

Expose an additional function, bind.WithFlag(), which allows callers to
use the previously-default "global flag" mode.

This change allows the bind string parser (etc.) to be used without
unwanted side effects. The behavior of the top-level "goji" package has
not been changed.

Fixes #47.

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.

Fixes #7

I've currently written three:
- Request ID generation
- Request logging, with color (!)
- Recovery from panics

The idea is to make goji super easy to use out of the box.