Add support for multiple IDs on a single call

Summary:
This diff adds support for querying multiple users at the same time using the
phabulous.fromslack and phabulous.toslack conduit methods.

Additionally, some style fixes were made to comply with phutil conventions.

Test Plan: N/A

Reviewers: etcinit

Reviewed By: etcinit

Differential Revision: https://phabricator.chromabits.com/D1