Bulk sending messages in single API call

It would be good to send multiple messages to the API in a single HTTP request. The current `send/message` and `send/raw` methods are designed for sending a single method.

A new method (or methods) will be needed for this purpose.

In theory, the API should simply accept an array of hashes containing the params that would normally be passed to the `send/message` or `send/raw` methods. It should then go through this array and compile a result hash that is the same format of that in the single messages above.

Start by locating the API entry points for `send/message` and `send/raw` and read how their single-message results are assembled. Define how an array of equivalent parameter hashes should be accepted and how the combined result should preserve the existing single-message result format. Done means one HTTP request can process multiple messages through the API.

Written by the indexing model from the issue text.