Record Class injection

This is the simplest possible implementation. There are a lot of things wrong with the way we do readers and writers, but I'm not sure this is the best time to tackle that (in lieu of more substantive changes for a backwards-incompatible 2.0 release or something). 

```ruby
    reader = MARC::Reader.new('test/one.dat', :record_class => MyRecord)
    reader.first.class #=> MyRecord

```