Moved all adapter specific code into adapters/ directory and updated all adapters to utilize the same "require_adapter" method of loading specific-database adapters.

* Adds back in SQLite3 and PostgreSQL support

issue-url: http://github.com/zdennis/activerecord-import/issues#issue/1 
[contributes #1]

test/active_record/connection_adapter_test.rb

@@ -1,6 +1,6 @@
 require File.expand_path(File.dirname(__FILE__) + '/../test_helper')
 
-describe "ActiveRecord::ConnectionADapter::AbstractAdapter" do
+describe "ActiveRecord::ConnectionAdapter::AbstractAdapter" do
   context "#get_insert_value_sets - computing insert value sets" do
     let(:adapter){ ActiveRecord::ConnectionAdapters::AbstractAdapter }
     let(:base_sql){ "INSERT INTO atable (a,b,c)" }