add _write_connection property to support replicated databases in Model_Crud

Signed-off-by: P. Isaac Hildebrandt <king.isaac@gmail.com>
add unit test for Model_Crud::get_connection()

Signed-off-by: P. Isaac Hildebrandt <king.isaac@gmail.com>
Merge pull request #1279 from ihildebrandt/1.5/connections

Added _write_connection property to Model_Crud to support replicated database setups