codegangsta/martini now becames go-martini/martini

Changed martini import path: 

<pre>
 import (
        "encoding/base64"
-       "github.com/codegangsta/martini"
+       "github.com/go-martini/martini"
        "net/http"
        "strings"
 )
</pre>
see also [ Panic when Using Render Handler #217 ](https://github.com/go-martini/martini/issues/217)