add support for Pipelines API

https://docs.gitlab.com/ee/api/pipelines.html

Covers all the api methods in docs except delete (there are no delete methods for other types, either).

A couple fields on struct Pipeline are undocumented since I wasn't sure of their meaning.

Still needs to be tested against a Gitlab instance; we're still running v11.7 at my shop, but if any has access to a v12 with pipelines, here's a test program:

//! Exercise the Pipeline API.
//!
//! NOTE: this will create, retry, and cancel a pipeline on the target
//!       project/branch.

extern crate gitlab;
use gitlab::{Gitlab};

fn main() {
    let empty_params: &[(&str,&str)] = &[];

    let client = Gitlab::new("HOST", "TOKEN")
        .expect("could not get gitlab client");

    let project = client.project_by_name("YOUR PROJECT", empty_params)
        .expect("could not get project");

    let pipelines = client.pipelines(project.id, empty_params)
        .expect("could not get pipelines");
    eprintln!("{:#?}", pipelines);

    let commit = client.commit(project.id, pipelines[0].sha.value())
        .expect("could not get commit");
    eprintln!("{:#?}", commit);

    let pipeline = client.pipeline(project.id, pipelines[0].id).expect("could not get pipeline");
    eprintln!("{:#?}", pipeline);

    let pipeline = client.create_pipeline(project.id, pipeline.sha, &[]).expect("could not create new pipeline");
    eprintln!("postcreate: {:#?}", pipeline);

    let pipeline = client.cancel_pipeline(project.id, pipeline.id).expect("could not cancel new pipeline");
    eprintln!("postcancel: {:#?}", pipeline);

    let pipeline = client.retry_pipeline(project.id, pipeline.id).expect("could not retry cancelled pipeline");
    eprintln!("postretry: {:#?}", pipeline);

    let pipeline = client.cancel_pipeline(project.id, pipeline.id).expect("could not cancel again");
    eprintln!("postcancel: {:#?}", pipeline);
}