Implement default fixtures.

The calling of default fixtures is now less cumbersome. 

When you instantiate the manger, you can pass in the default model type, and the YAML fixtures will be called.
`manager->loadFixture('customer/customer');`

You can also specify the full path of the fixture file, if it needs to be user defined
`manager->loadFixture('customer/customer', '/path/to/customer.yml');`