feat(client): add a new Client struct with super powers

- Includes ergonomic traits like IntoUrl and IntoBody, allowing easy
usage.
- Client can have a RedirectPolicy.
- Client can have a SslVerifier.

Updated benchmarks for client. (Disabled rust-http client bench since it
hangs.)

Closes #62 
Instead of building this `RequestOptions` struct, which will break if we add fields in the future, I think it would be best if `client.request` returned a builder type.