Add expectation for the current path

[matthewmcgarvey]

Fixes #94

This allows us to write specs like

it "requires signed in users" do
  flow = BaseFlow.new
  flow.visit Authenticated::Endpoint::Index
  flow.should have_current_path(SignIn::New)
end

[jwoertink]

Nice! I dig it.

[paulcsmith]

This is super cool! Nice idea and implementation

[paulcsmith]

Sorry to come in late, but could you make a PR that also tests with a Lucky::Action class to make sure the other two method overloads work?

[matthewmcgarvey]

We can't write a test for those because this library doesn't actually depend on lucky. If I called those methods it would fail to compile.