Add channel members API

[alinavancea]

Issue #33

Summary of the changes

• Add members method to under channel.rb

• Add docs

• Add rspec tests

• rspec is passing

• rubocop has some offenses

41 files inspected, 52 offenses detected, 1 offense auto-correctable

[abrom]

FYI this has been released in v0.1.20

[alinavancea]

Thank you @abrom 🙇‍♀️