Improve `debugger` so that an IRB session is started when ruby-debug is not loaded.

This is just crazy, random idea I had today.  I'm looking for feedback more than anything at this point.

I frequently receive the `use -d or --debug` warning, for a few reasons:

* At work, I'm building lots of new small, internal gems, and I don't pull in a debugger gem as a dependency until I really need it...so I'll be working on something, and will put `debugger` in a spec, and won't realize it can't work yet.
* Occasionally, I need to debug a spec failure on our CI server, where we usually `bundle install --without debugger`--so ruby-debug isn't installed.
* For a while after I started using 1.9.3, there wasn't a viable debugger gem to use. There still isn't a 1.9.3 compatible release of `ruby-debug`. cldwalker/debugger has dealt with this problem and I use that now, but it was frustrating to not have a debugger for a while.  It may happen again (e.g. with the release of ruby 1.9.4 or 2.0).

90% of the time, I don't need the power of a full debugger--I just want to poke around in an IRB session at a particular point in the code.  It turns out, we can easily provide that functionality directly in rspec for cases when a debugger is not loaded.  So...this is a first pass implementation that makes our `debugger` print a message and start an IRB session with `self` set to the receiver of `debugger`.  Here's what the output looks like:

```
*****************************************************************
A `debugger` statement has been encountered but ruby-debug is not
loaded. For a full debugger, install `ruby-debug` and use the -d
or --debug option.

The `debugger` statement was encounted at:
/Users/myron/code/rspec-dev/repos/rspec-core/spec/rspec/core/example_spec.rb:21:in `block (3 levels) in <top (required)>'

      16    end
      17  end
      18
      19  describe "#exception", :focus do
      20    it "supplies the first exception raised, if any" do
 =>   21      debugger
      22      example = example_group.example { raise "first" }
      23      example_group.after { raise "second" }
      24      example_group.run
      25      example.exception.message.should eq("first")
      26    end

We've setup an IRB sessionso you can poke around here
a bit. Type `exit` to continue.
*****************************************************************
```

A couple thoughts/concerns I have:

* A debugger doesn't really belong in rspec-core.  Adding this opens up the door to people requesting additions to it (e.g. `next`, `step`, up/down the callstack, etc), but I think it's important we don't go down that route.  What's hear is appealing to me because it's just IRB, which is already in the stdlib.
* This has the potential to break non-interactive spec runs (e.g. on a CI server or whatever).  Before, `debugger` would just print a warning and continue happily on its way...now it stops and waits for user input.  Is there a way to tell if rspec is running in an interactive terminal?
* There are some extra edge cases this needs to handle (e.g. `listing` needs to handle source files that have less than 11 lines of code).  (I'm mostly mentioning this so I don't forget, if we do decide to merge this).

Thoughts?