Overriding Variables

Support for overriding variables inside the FactoryGroup while creating it.

Example:
```ruby
described_class.define(:user_group) do
  user FactoryGirl.create(:user)
end
```
We should be able to do something like the following.
```ruby
overriding_user = FactoryGirl.create(:user, :name => "Another User")
FactoryGroup.create(:user_group, :user => overriding_user)
```