Add session API

[kzaitsev]

Add ability to call session.

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)
}

[plouc]

@Bugagazavr, sorry for the (very, very…) long delay, can you please add some tests on this PR ?

[kzaitsev]

@plouc hello, gitlab now works as oauth2 provider, currently im not need this method.