Raise an error with a helpful message when Instruments.app is open

If the Instruments.app is open, the instruments command line tool cannot take control of the application.

RunLoop.run detects if Instruments.app is running and raises a helpful message.

```
$ irb
Your wish is my command.
irb(main):001:0> RunLoop.run
RuntimeError: Please quit the Instruments.app.
If Instruments.app is open, the instruments command line
tool cannot take control of your application.
	from /Users/moody/git/run-loop/lib/run_loop/core.rb:653:in `run'
	from (irb):1
	from /Users/moody/.rbenv/versions/2.1.3/bin/irb:11:in `<main>'
irb(main):002:0> 
```