Missing support for next_cursor

[alexherreid]

At this time, there is no implementation of the next_cursor in response_metadata, meaning for calls that are limited (or can be limited) in response, there's no way to get the rest of the data. For example, having over 100 groups/private channels, or over 1000 users can mean no access to the full data set.

[alexherreid]

#231 is the resolve for this