deprecate `assert_blank` and `assert_present`.

They don't add any benefits over `assert object.blank?` and `assert object.present?`

I did:

1. add deprecation warnings to `assert_blank` and `assert_present`
1. replaced every `assert_blank` and `assert_present` call in the rails code base
1. added a CHANGELOG entry
1. added the deprecation to the rails 4 guide