fix bug #506 - .uniq breaks with pg and json columns

fix bug #506 - .uniq breaks with pg and json columns

updated specs for 'select distinct' and pg issues

* new model/table with a json column for postgresql specific testing
* add tests for explicit distinct in a select and for all queries to
  test if tagged_with adds a distinct clause on the query.
  http://github.com/mbleigh/acts-as-taggable-on/issues/357
* add tests for taggable_model_with_json
* exclude test should sort the array result and expected array.
  PostgreSQL won't always return the two expected results in
  the expected order so the test failed intermittently.

use group vs distinct when options[:any] is true

update CHANGELOG

check pg version and only use json if >=9.2

move change to master section

move table with json to spec/internal/db/schema.rb