Added support for cursor pagination, closes #157.

[dblock]

Using the same methods you can do:

members = []
client.users_list(presence: true, limit: 5) do |json|
  expect(json.ok).to be true
  members.concat json.members
end

which translates to the following in methods that support cursor

Pagination::Cursor.new(self, :users_list, options).each do |page|
  yield page
end

[dblock]

@alexagranov Would love a PR and merge/comments, please!

[dblock]

I'll just merge it, feel free to add comments later @alexagranov ;)

[alexagranov]

Sorry @dblock, totally missed this. Looks great!

Next time try tagging me as a Reviewer?

[dblock]

Will do, I was mostly just being impatient.