Improve performance of read_attribute

The modifications injected into the AR::Base class for read/write_attribute cause a performance slowdown on all ActiveRecord attribute reads. This refactoring removes the Railtie which includes BelongsToHstore in AR::Base so that classes without hstore attributes are not impacted.

This is a breaking change and requires that the BelongsToHstore is included into any class that uses it.

We've seen a 5-10% speed improvement across the board with this change and a reduction in the number of Strings created.