Usable word count feature

I think the word count feature is good enough to merge to master at this point.  

By default it will only scan the message header because scanning the full message doesn't quite scale up to the full enron depot.  It's almost there, but the Executing Words -> select word -> Dates takes about a minute to complete for the words with highest counts.  

I know I could merge the changes directly, but I'd rather it go through a code review.  Plus the core.js.min is a sym-link in my branch.  We should add to README the steps needed to process the js code.