Relicense to dual MIT OR Apache-2.0 #23

LICENSE and README currently say MIT-only, but Cargo.toml has been MIT OR Apache-2.0. We should make the files match and dual-license the repo MIT OR Apache-2.0.

This licensing scheme gives maximum flexibility for downstream users. Apache 2.0 includes an explicit patent grant. The user may choose either license.

To treat prior contributions as Apache-2.0 as well as MIT, we need explicit approval from everyone with a meaningful contribution.
If you're on the list below and agree, please comment:

I hereby re-license my previous contributions to electrum_streaming_client under Apache 2.0 in addition to the existing MIT license.

• @evanlinjin
• @noahjoeris
• @oleonardolima