Refactoring and enhancements

Hi,
first of all, thanks for sharing your library, i use it in a very little command line to clean my twitter account :+1: 
 
My comments : 
1. SSTwitterApi is about 5000 lines, i think it too much. To have a better structure of your library, i split your API by functionality. Good idea, isn't it ?
2. i've seen your not checking the availability of the completionBlock when you use it, i think a macro is usefull and keep code clean (ST_BLOCK_SAFE_RUN). I think this job has to be done in the entire project, i'have done it in (STTwitterAPI, STTwitterAPI+Timelines, STTwitterAPI+Tweets, STTwitterAPI+Lists)