Better top-level AppEngine support
Add BytesWritten() method to WriterProxy.

This makes it possible to show content length when writing the access log.
Merge pull request #69 from mredivo/master

Add BytesWritten() method to WriterProxy.
Refactor package graceful

This is meant to accomplish a few things:
1. graceful no longer spawns an additional goroutine per connection.
   Instead, it maintains a sharded set of idle connections that a single
   reaper goroutine can go through when necessary.
2. graceful's connection struct has a more orthogonal set of connection
   state flags, replacing the harder-to-understand state machine. The
   underlying mechanics are largely the same, however.
3. graceful now uses the Go 1.3 ConnState API to avoid the "200-year
   SetReadDeadline hack." It still falls back on SetReadDeadline on Go
   1.2 or where ConnState does not apply.
Add bind.Sniff to expose default selection logic

This allows users who do not wish to use bind.WithFlag() to still take
advantage of bind's magical listener selection logic.

Fixes #68.