message: feat(client): add client::conn::Builder::executor method
diff: use std::fmt;
  use std::marker::PhantomData;
  use std::mem;
  +use std::sync::Arc;
  use bytes::Bytes;
  use futures::{Async, Future, Poll};
  -use futures::future::{self, Either};
  +use futures::future::{self, Either, Executor};
  use tokio_io::{AsyncRead, AsyncWrite};
  use body::Payload;
  @@ -441,6 +442,15 @@ impl Builder {
  self
  }
  + /// Provide an executor to execute background HTTP2 tasks.
  + pub fn executor<E>(&mut self, exec: E) -> &mut Builder
  + where
  + E: Executor<Box<Future<Item=(), Error=()> + Send>> + Send + Sync + 'static,
  + {
  + self.exec = Exec::Executor(Arc::new(exec));
  + self
  + }
  +
  pub(super) fn h1_writev(&mut self, enabled: bool) -> &mut Builder {
  self.h1_writev = enabled;
  self
  