Integrate with futures-rs

[brunocodutra]

The goal of this PR is to make it convenient to asynchronously dispatch actions against a store that is running on a background thread[pool]:

#![feature(async_await, await_macro, futures_api)]
use reducer::*;

let store = AsyncStore::new(Calculator(0), Display);

// Process incoming actions on a background thread.
let mut dispatcher = store.spawn_thread().unwrap();

let compute = async {
    await!(dispatcher.dispatch(Action::Add(5)));
    await!(dispatcher.dispatch(Action::Mul(3)));
    await!(dispatcher.dispatch(Action::Sub(1)));
    await!(dispatcher.dispatch(Action::Div(7))); 
};

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@4cdb9e3). Click here to learn what that means.
The diff coverage is 90.14%.

@@            Coverage Diff            @@
##             master      #11   +/-   ##
=========================================
  Coverage          ?   95.62%           
=========================================
  Files             ?       18           
  Lines             ?      274           
  Branches          ?        0           
=========================================
  Hits              ?      262           
  Misses            ?       12           
  Partials          ?        0

Impacted Files	Coverage Δ
src/dispatcher/mod.rs	100% <ø> (ø)
src/dispatcher/store.rs	100% <ø> (ø)
benches/async.rs	0% <0%> (ø)
src/dispatcher/async_store.rs	95.52% <95.52%> (ø)

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[brunocodutra]

Tarpaulin seems to be segfaulting on thread related unit tests - xd009642/tarpaulin#190