Add session API

Add ability to call session.

```go
package main

import (
	"fmt"
	"github.com/plouc/go-gitlab-client"
)

func main() {
	client := gogitlab.NewGitlab("https://gitlab.com", "/api/v3", "")

	session, err := client.GetSession("admin@example.com", "ExamplePass")
	if err != nil {
		fmt.Println(err)
	}

	fmt.Println(session.PrivateToken)
}
```