Makes terms_for_display accesible on Worthwhile::GenericFile

This is so that the :title attribute (along with any other terms) on GenericFile create/update form actually gets applied to the GenericFile object

refs #34
This causes a deprecation warning:
```
.DEPRECATION WARNING: You attempted to set the attribute `title' on `Worthwhile::GenericFile' to a scalar value. However, this attribute is declared as being multivalued. This behavior is deprecated and will raise an ArgumentError in active-fedora 8.0.0. (called from block in attributes= at /home/travis/.rvm/gems/ruby-2.1.4/gems/active-fedora-7.1.1/lib/active_fedora/attributes.rb:21)
```
Can you set it to an array instead?