Added support for create_topic, delete_topic, set_topic_attributes, subs...

...cribe apis

I have followed the style of the erlcloud_sns.erl module, but I have to say that the way that erlcloud_sns returns results is different from the rest of the erlcloud library where most APIs return {ok, Result} | {error, Reason} tuple, the erlcloud_sns exists the process.... 
While I don't object to exiting the process I want consistency...
Would you like me to modify erlcloud_sns to be consistent with the rest of the APIs?