add blocking profile handler

Example usage:

```
package main

import (
    "net/http"
    "time"

    "github.com/codahale/http-handlers/debug"
    )

func main() {
    ch := make(chan int)
    go func() {
        for {
            ch <- 1
        }
    }()
    go func() {
        for {
            select {
                case <- ch:
                default:
                    time.Sleep(1)
            }
        }                                                                                                                                                                                                                                    
    }()
    http.ListenAndServe(":8080", debug.Wrap(http.HandlerFunc(http.NotFound)))
}
```

```
$ go tool pprof http://localhost:8080/debug/pprof/block
Read http://localhost:8080/debug/pprof/symbol
Fetching /pprof/block profile from localhost:8080 to
  /var/folders/3k/xcbr5k6s4f5ddvdjv0xd87z00000gn/T/4fJElFuB9K
Wrote profile to /var/folders/3k/xcbr5k6s4f5ddvdjv0xd87z00000gn/T/4fJElFuB9K
Welcome to pprof!  For help, type 'help'.
(pprof) top10
Total: 19.604 seconds
  19.604 100.0% 100.0%   19.604 100.0% runtime.chansend1
   0.000   0.0% 100.0%   19.604 100.0% main.func·001
   0.000   0.0% 100.0%   19.604 100.0% runtime.goexit
```

I have no idea how to unit test this.