Allow batching multiple events with different payloads

[Diggsey]

The HTTP API has no restriction on the payload, but the Rust API only allows batching with a singe payload being sent to all channels.

[countradooku]

@Diggsey try my crate https://github.com/RustNSparks/pusher-http-rust . it is not a fork. it is write form scratch and it is feature complete