This is a relatively large release of the `futures` crate, although much of it
is from reworking internals rather than new APIs. The banner feature of this
release is that the `futures::{task, executor}` modules are now available in
`no_std` contexts! A large refactoring of the task system was performed in
PR #436 to accomodate custom memory allocation schemes and otherwise remove
all dependencies on `std` for the task module. More details about this change
can be found on the PR itself.

Other API additions in this release are:

* A `FuturesUnordered::push` method was added and the `FuturesUnordered` type
  itself was completely rewritten to efficiently track a large number of
  futures.
* A `Task::will_notify_current` method was added with a slightly different
  implementation than `Task::is_current` but with stronger guarantees and
  documentation wording about its purpose.
* Many combinators now have `get_ref`, `get_mut`, and `into_inner` methods for
  accessing internal futures and state.
* A `Stream::concat2` method was added which should be considered the "fixed"
  version of `concat`, this one doesn't panic on empty streams.
* An `Executor` trait has been added to represent abstracting over the concept
  of spawning a new task. Crates which only need the ability to spawn a future
  can now be generic over `Executor` rather than requiring a
  `tokio_core::reactor::Handle`.

As with all 0.1.x releases this PR is intended to be 100% backwards compatible.
All code that previously compiled should continue to do so with these changes.
As with other changes, though, there are also some updates to be aware of:

* The `task::park` function has been renamed to `task::current`.
* The `Task::unpark` function has been renamed to `Task::notify`, and in general
  terminology around "unpark" has shifted to terminology around "notify"
* The `Unpark` trait has been deprecated in favor of the `Notify` trait
  mentioned above.
* The `UnparkEvent` structure has been deprecated. It currently should perform
  the same as it used to, but it's planned that in a future 0.1.x release the
  performance will regress for crates that have not transitioned away. The
  primary primitive to replace this is the addition of a `push` function on the
  `FuturesUnordered` type. If this does not help implement your use case though,
  please let us know!
* The `Task::is_current` method is now deprecated, and you likely want to use
  `Task::will_notify_current` instead, but let us know if this doesn't suffice!