Multipart Support

I've added some basic multipart form support. It works, but the API is (IMO) a little rough but could benefit from some feedback. 

Here's a full usage example for uploading a file, and here's a link to view the requests it produces http://requestb.in/nkp6grnk?inspect
```go
package main

import (
        "github.com/franela/goreq"
        "io/ioutil"
        "os"
        "path"
)

func main() {
        // The Gutenberg project is *awesome*
        // http://www.gutenberg.org/ebooks/45269
        fileName := "Inventions_of_the_Great_War--A.Russell_Bond.mobi"
        file, err := os.Open(path.Join("/tmp", fileName))
        if err != nil {
                panic(err)
        }       
        
        defer file.Close()
        
        body := goreq.MultipartForm{
                Params: map[string]string{
                        "title":       "Inventions of the Great War",
                        "author":      "A. Russell Bond",
                        "description": "A Gutenberg project book originally found at http://www.gutenberg.org/ebooks/45269",
                },      
                Field:    "file",
                FileName: fileName,
                Source:   file,
        }       
        
        resp, _ := goreq.Request{
                Uri:          "http://requestb.in/nkp6grnk",
                MaxRedirects: 5,
                Method:       "POST",
                Body:         body,
        }.Do()  
        
        println(resp.StatusCode)
        b, _ := ioutil.ReadAll(resp.Body)
        println(string(b))
}
```