Rename `ignore` to `transient`

FactoryGirl has a feature called "Transient Attributes" that allows factories to define attributes that can be used inside the factory, but are not to be persisted to the model. The API for this has been to pass a block to a method called `ignore`. The name `ignore` is inconsistent with the feature name of "Transient Attributes".

This pull request deprecates the `ignore` method, and creates a new `transient` method with the same functionality.

One concern is that the internal API for querying transient attributes is still called `ignored` and `non_ignored`. This seems inconsistent, but that is up for debate.