implement marc382 parsing

Fixes #268 

Sets up a pattern but not generic classes to add custom marc parsing outside of helpers.

## 10246987
![screen shot 2014-08-06 at 3 47 37 pm](https://cloud.githubusercontent.com/assets/1656824/3835242/d1cf5798-1dbb-11e4-915d-ddd1d0928184.png)

`#try` will return nil if it's not able to call map on `@subfields`.  This means it's possible to call `each_with_index` on `nil` below.  I'm just trying to understand the case for using `#try` here.