ZestLoops

This pull request contains all the code for managing loops inside Zest. It consists on 3 typre of loops:
1- LoopString: loops through a set of String;
2- LoopInteger: loops through a set of int;
3- LoopFile: loops through a set of String contained in a file.

I tried to maintain the usage of loops as easy as possible, for this reason they implements Enumeration<ZestStatement>.