Switch MR to use ActiveRecord's `[]` and `[]=` attribute methods

This switches MR to use ActiveRecord's hash reader and writer
methods. This should be allow more flexibility with pulling back
custom fields using a `SELECT` statement in SQL.

Closes #15
Merge pull request #19 from redding/jcr-use-hash-reader-writer

Switch MR to use ActiveRecord's `[]` and `[]=` attribute methods