Anatoly I

Results 5 issues of Anatoly I

This PR adds support of a custom executors (defaults to `tokio::executor::DefaultExecutor`). Required bounds for supported executors are defined as `MyExecutor` trait. New constructors: * `Conn::with_executor` * `Conn::from_url_with_executor` * `Pool::with_executor` *...

This commit also updates `percent-encoding` dependency.

This PR converts package to an ECMAScript module. Diffs are noisy because of a Prettier, sorry.