add gin benchmark

Hi Matthew,

I added a Gin (https://github.com/gin-gonic/gin) based benchmarker to the project. Gin is a more Go idiomatic Sinatra style microframework for Go. I think it would be a good addition to this showdown.

On my machine I got the following results:

```
$ go get github.com/gin-gonic/gin
$ GOMAXPROCS=4 MARTINI_ENV=production go run server.go
[GIN-debug] GET   /:title                   --> main.func·001 (2 handlers)
[GIN-debug] Listening and serving HTTP on :3000

$ ./wrk -t4 -c100 -d30S --timeout 2000 "http://127.0.0.1:3000/showdown"
Running 30s test @ http://127.0.0.1:3000/showdown
  4 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     2.83ms    3.62ms  57.75ms   97.92%
    Req/Sec    10.77k     1.91k   14.44k    82.29%
  1220584 requests in 30.00s, 2.36GB read
Requests/sec:  40686.61
Transfer/sec:     80.40MB
```

Compared to Martini:

```
Running 30s test @ http://127.0.0.1:3000/showdown
  4 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     8.77ms    4.55ms  71.54ms   82.16%
    Req/Sec     2.91k   239.18     4.31k    72.46%
  342695 requests in 30.00s, 682.07MB read
Requests/sec:  11424.21
Transfer/sec:     22.74MB
```