Add Middleware.Timeout

[pbrisbin]

See #701.

Wraps the inner respond in System.Timeout.timeout with the given seconds
and, by default, responds status503. The main timeout is implemented in
terms of timeoutStatus for customizing only the response status, which is
itself implemented in terms of timeoutAs, for customizing the Response
entirely.

Some things I'm not sure of:

1. timeout is a bit of a bold name, but it seems in line with how all
   Middlewares name themselves: assuming a very narrow context
2. Is timeoutStatus useful, or is just timeout/timeoutAs enough?
3. If we keep timeoutStatus, should timeoutAs be named timeoutResponse,
   which feels more consistent?
4. Should we accept microseconds directly? Is there ever a use-case for a
   timeout smaller than one second? (it would be nice to not have to wait an
   entire second for each of the spec examples)

Any other tweaks or style changes, don't hesitate to let me know.

[snoyberg]

LGTM! Can you give this a minor version bump in the cabal file, fill in the @since comments in the module for all exported identifiers and the module itself, and add a changelog entry?

[pbrisbin]

Will do!

[pbrisbin]

@snoyberg this should be all set now :)

[snoyberg]

LGTM, thanks!