`None` -> `Ok(None)`

Refs: https://github.com/rust-lang-nursery/wg-net/issues/54

This also bumps the following sub crate versions:

* tokio-current-thread (0.1.4)
* tokio-reactor (0.1.7)
* tokio-signal (0.2.7)
* tokio-threadpool (0.1.9)
* tokio-timer (0.2.8)
* tokio-udp (0.1.3)
* tokio-uds (0.2.4)

`inner` is a fitting name for variables of type named `Inner`, but in other cases I find them confusing - sometimes `inner` refers to a `Pool`, sometimes to a `Sender`. I renamed a bunch of variables named `inner` to be more descriptive.

This PR is the first step in an effort of splitting https://github.com/tokio-rs/tokio/pull/722#issuecomment-439552671 into multiple PRs.

Implement `Stream + Sink` layer on top of unix domain sockets
using codecs.

When spawning using `Handle` while on the executor, tasks were being
double counted. This prevented the number of active tasks to reach zero,
thus preventing the executor from shutting down.

This changes `spawn` to check if being called from the executor
**before** incrementing the number of active tasks.

Fixes #760

Should hopefully fix the underlying bug that was causing tokio-tls tests to occasionally fail on Windows.
Signed-off-by: Toby Lawrence <toby@nuclearfurnace.com>

Throttle down a stream by enforcing a fixed delay between items.

* tests: allow nightly builds to fail
Signed-off-by: Toby Lawrence <toby@nuclearfurnace.com>

This file is testing deprecated code, so it should be permitted to
access deprecated code.

Closes #705

* async-await: fix README example dependencies

As per commit "async-await: track nightly changes (#661)" ( commit
2f690d30)

> The `tokio-async-await` crate is no longer a facade. Instead, the
> `tokio` crate provides a feature flag to enable async/await support.

Ensure the example in the async-await README file also works by
correctly declaring this updated dependency

* async-await: remove unnecessary 'edition' declaration from README

As the "edition" feature was stabilized in rust v1.30 and async-await
specifies that the nightly toolchain must be used, remove the use of the
"edition" feature gate since it is enabled by default.