Stop testing on 1.8.7, 1.9.2, rbx-18mode, and rbx-19mode.
Adding Rails 4 and its dependent gems.

Unfortunately we can't just use only activerecord, since it's not released as stand alone gem.

Also:
* Replacing all .scoped calls with .all
* Using new order syntax on AR associations
* Replaced deprecated finder methods
* Using to_a for comparing activerecord relations in specs.
* Using correct activerecord syntax in nested_set_scope method
Use only activerecord, not all of Rails.
attr_accessible was removed from activerecord.
Let specs run and fix deprecation warnings about default_scope.
Delegate connection to class

Rails 4 has deprecated the instance connection method, this adds a delegation method back to skip the deprecation warning
Merge pull request #184 from lexrupy/rails4

Delegate connection to class
Fix ordering for left_sibling
Remove left_sibling order clause completely.

This is already handled by siblings via nested_set_scope
Refactor into existing or new methods where possible.
without_self returns scope when not persisted.

Fixes #157
pulled out the columns and model modules into their own files, and pulled out the move_to logic into its own object
pull out pruning logic into its own module and split up complex method
pull out moving logic into its own module and break up complex methods
move rebuild into its own class and simplify code
move complex iteration methods into separate module (one of them has no tests, and it doesn't seem to behave correctly but I don't know enough about it to know for sure)
refactor complex sql conditions
move retry transaction logic into module, and moved set validation sql query into class and replaced strings with Arel.
move require statements into the file that needs them
don't need entire namespace, and include an earlier commit that got deleted by a rebase