Refactor the core classes

(e.g., not the readers and writers)

I just went through the core classes and refactored them in a more modern Ruby style, pulling out a couple methods and basically just cleaning things up. The change to require minitest4 was just to get the tests to run at all under ruby 2.1.

Sorry about the whitespace, but no so sorry that I'm going to try to do it all again.

Major types of changes:
* Use Forwardable to forward enumerators and a few other things to contained subfields, fields
* Reduce the complex if/then logic of comparator (`def ==(other)`) methods to just do the comparisons
* a couple instances of `alias_method` where that's all that was happening
* major simplification of the marc-in-json roundtrip code


Comments welcome.