[WIP] Adding some initial command functions

Work in progress commit that will add three helper functions to the Command package:

1. `createRequest()` - Does the first half of a command by creating a request for the command. The request created for the command is then returned.
2. `getCommandResponseStatus()` - Executes a command and only returns the HTTP response status code of the response received when executing the command. Null is returned if an error occurs or no response is received.
3. `batch()` - Executes one or more commands in parallel and returns a hash of commands mapped to their result (or exception). This is analogous to the `batch()` function in Guzzle: https://github.com/guzzle/guzzle/blob/master/src/functions.php#L138.

I'm still working on the API, method names are still up in the air, need tests, and could potentially remove one or more of these functions depending on feedback.