Add ActiveFedora::Relation#last. Resolves #184

[acurley]

Provides convenience method to determine the last ActiveFedora object based on PID.

[jcoyne]

Why are we setting @last if that variable is never used?

[acurley]

I was following the pattern for #first. Should I add @last to the reset method as well? I can't find when @first is actually invoked in a running app.

[jcoyne]

I guess I didn't see the ||= operator, that uses it after its defined. However, you probably want to clear the cache here: https://github.com/APTrust/active_fedora/blob/bbb77d8fe309b905859ec644abc8017699507981/lib/active_fedora/relation.rb#L17