Breaking change.

* Implement a new module `connection`, which abstracts `std::net` types into ones that can use
  `std::os::unix::net` types on Unix platforms.
* Change `ServerConfig::addr` to a new, abstracted type and add `http_unix` method.
  `http` and `https` methods are unchanged and should still work.
* `Request::remote_addr` now returns `Option<&SocketAddr>`. This is `Some` for TCP servers and
  `None` for UNIX servers (since UNIX remote sockets are almost always unnamed).

Some valid versions of log 0.4.x don't work with Rust 2018 idioms so
update the version specifier here to require at least 0.4.4

This PR allows the dependency on `zeroize` to float amongst whatever stable version might be in use by the project including this library. Not allowing it to float leads to incompatibilities with widely-used crates like `x25519-dalek` and `rsa`, which have older versions on pre-1.5 versions of `zeroize`.

With 'static, one can only use static string slices, which makes it
impossible to test with urls generated at runtime, such as ones
generated by a fuzzer.

This change is backwards-compatible, because it only relaxes the
requirements on `with_path`, and although the type of the field changed,
that field is not exposed.

This doesn't incur an extra copy either. The copy was already there,
it only got moved out of `into` and into `with_path` now.

put the test back

format

Reduce dependencies, switch to `httpdate` crate