Add named subject.

As discussed with @zenspider at GoGaRuCo.

RSpec allows a name to be given when declaring a subject. This works exactly like simply declaring a `let` with the same name.  This PR adds that functionality.

Background information:

http://betterspecs.org/#subject
http://blog.davidchelimsky.net/blog/2012/05/14/spec-smell-explicit-use-of-subject/ (in Update 2012-05-15 section at bottom)