feat(server): Implements RunFd method

[yyoshiki41]

Renewed the pr #526.

---

I use gin http.Server under circus which is a socket manager.
I need to run http.Server through a file descriptor, so implemented RunFd method.

When using other socket managers (e.g. einhorn), maybe work well.

Usage

package main

import (
    "flag"
    "github.com/yyoshiki41/gin"
)

var (
    fd = flag.Int("fd", -1, "")
)

func main() {
    flag.Parse()

    r := gin.Default()
    // Ping test
    r.GET("/ping", func(c *gin.Context) {
        c.String(200, "pong")
    })
    // Method arg is the file descriptor number(int).
    r.RunFd(*fd)
}

and, circus.ini example is below.

[circus]
statsd = 1

[watcher:webapp]
cmd = ./main -fd $(circus.sockets.web)
numprocesses = 1
use_sockets = True

[socket:web]
host = 127.0.0.1
port = 8080

Demo

$ circusd circus.ini
2016-02-03 11:07:13 circus[46347] [INFO] Starting master on pid 46347
2016-02-03 11:07:13 circus[46347] [INFO] sockets started
2016-02-03 11:07:13 circus[46347] [INFO] Arbiter now waiting for commands
2016-02-03 11:07:13 circus[46347] [INFO] webapp started
2016-02-03 11:07:13 circus[46347] [INFO] circusd-stats started
[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
 - using env:   export GIN_MODE=release
 - using code:  gin.SetMode(gin.ReleaseMode)

[GIN-debug] GET    /ping                     --> main.main.func1 (3 handlers)
[GIN-debug] Listening and serving HTTP on fd@5
2016-02-03 11:07:13 circus[46354] [INFO] Starting the stats streamer

# Ping test
$ curl localhost:8080/ping
pong

# Other terminal
[GIN] 2016/02/03 - 11:08:11 | 200 |    2.571629ms | 127.0.0.1 |   GET     /ping

thx ☀️

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This change is 

[codecov]

Codecov Report

Merging #1609 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1609      +/-   ##
==========================================
+ Coverage   99.28%   99.28%   +<.01%     
==========================================
  Files          40       40              
  Lines        1948     1958      +10     
==========================================
+ Hits         1934     1944      +10     
  Misses         10       10              
  Partials        4        4

Impacted Files	Coverage Δ
gin.go	100% <100%> (ø)	⬆️

---

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