Add getItem() method to Table and modify find() to return getItem().then(rejectIfUndefined)

I get why you want to follow DynamoDB's behavior, but it's not very helpful to me as a developer using the library to have `find()` resolve when it wasn't actually able to find the thing I requested. Also, since the method names on Table (`find`) don't actually match up with the ones on DynamoDB (`getItem`), it seems to me that there isn't a necessity to align their behavior exactly with DynamoDB's methods. I've also attempted to utilize CoffeeScript's syntactical sugar more inside Table.coffee and aws-translators.coffee.