Simplifying blocks

Removed slow functionality from the gem. In particular locks and filters by block. They are useless for the primary use case (because the whole reason this tool exists is to make large tables small), and I'd rather keep the tool simple and easily testable than have lots of optional functionality. 

@kristjan @andyvb please review. 

@kball FYI, not expecting you to do anything about it. 
Are we requiring ruby > 1.9 for this gem?