Verify documentation links on Travis

Let's use [`cargo-deadlinks`](https://github.com/deadlinks/cargo-deadlinks) with the following command on Travis:

```rust
cargo deadlinks --check-http
```

This currently fails with some errors which are due to re-exports from `std`:

```
Found invalid urls in /home/stjepang/work/async-std/target/doc/async_std/io/type.Result.html:
Linked file at path /home/stjepang/work/async-std/target/doc/async_std/result/enum.Result.html does not exist!
Found invalid urls in /home/stjepang/work/async-std/target/doc/async_std/io/struct.Error.html:
Linked file at path /home/stjepang/work/async-std/target/doc/std/io/struct.Error.html does not exist!
Linked file at path /home/stjepang/work/async-std/target/doc/std/io/enum.ErrorKind.html does not exist!
Linked file at path /home/stjepang/work/async-std/target/doc/async_std/ffi/struct.NulError.html does not exist!
```

The way we resolve these errors is by writing shim docs for re-exports from `std`, similarly to how we did that here:

https://github.com/async-rs/async-std/blob/addda3937fe62c892e28747dd5aa33ce339bb7fa/src/os/unix/net/mod.rs#L14

The idea is that under the `docs` feature flag we generate "fake" docs linking to `async-std`'s types, but otherwise re-export real types from `std`.