Added write_attribute method for attribute setter methods
*Allows similar use of 'write_attribute' in ActiveRecord for methods that override an attribute setter
*Same as ActiveRecord, write_attribute does not save the model instance
Fixes #100 which notes memory leak for NSDataDetector

Current fix uses a class variable for the NSDataDetector and
exposes it through the class method `DateParser.detector`.

A run through Instruments as suggested in the bug does not
reveal increasing memory consumption any more.
Merge pull request #120 from cognitiveflux/write_attribute

Added write_attribute method for attribute setter methods
Added arbitrary type feature for columns.
bumped version number