Compare documents with max bson size when serializing 

@durran let me know if you can think of a better way to check for max size when serializing.

I'm not sure adding a method [#size_limited?](https://github.com/mongodb/mongo-ruby-driver/pull/503/files#diff-5187c4deaf844e24f2a98c6a49bbfc38L145)  on a Document type and then using #respond_to? is the best way. Though I didn't want to change the API of each type to take in a max size as an argument to the #serialize method.