Update examples to use new Tokio (#316)

The old `tokio-core` crate is deprecated in favour of the `tokio` crate,
which also provides the new multithreaded Tokio runtime. Although `h2`
is generic over the runtime, the examples do depend on `tokio`. 

This branch update the example code to use the new `tokio` library
rather than `tokio-core`. It also updates the examples in `RustDoc`.

For the most part, this was pretty trivial --- simply replacing
`handle.spawn(...)` with `tokio::spawn(...)` and `core.run(...)` with
`tokio::run(...)`. There were a couple of cases where error and item
types from spawned futures had to be mapped to `(), ()`. Alternatively,
this could have been avoided by using the single-threaded runtime and
calling `block_on` instead to await the value of those futures, but I
thought it was better to reduce the amount of tokio-specific code in the
examples.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Cargo.toml

@@ -62,7 +62,7 @@ serde = "1.0.0"
 serde_json = "1.0.0"
 
 # Akamai example
-tokio-core = "0.1"
+tokio = "0.1.8"
 env_logger = { version = "0.5.3", default-features = false }
 rustls = "0.12"
 tokio-rustls = "0.5.0"