detect project and rails root

This is a first stab at implementing the project/rails root detection with proper error messages. It works but when the project or rails root can't be detected spring crashes. I wanted to show you the current implementation to start a discussion.

Currently if the rails root can't be detected it fails like:

```
          Spring was not able to locate the rails root of your project. You should:
            - change your woring directory.
            - configure the location of your rails application using `config/spring.rb`.

          Spring used the following paths to detect the Rails root (`config/application.rb`):
            pwd: /Users/senny/Projects/spring/test/apps
            project root: /Users/senny/Projects/spring
/Users/senny/Projects/spring/lib/spring/application_manager.rb:53:in `run': undefined method `chomp' for nil:NilClass (NoMethodError)
	from /Users/senny/Projects/spring/lib/spring/server.rb:39:in `serve'
	from /Users/senny/Projects/spring/lib/spring/server.rb:31:in `block in boot'
	from /Users/senny/Projects/spring/lib/spring/server.rb:31:in `loop'
	from /Users/senny/Projects/spring/lib/spring/server.rb:31:in `boot'
	from /Users/senny/Projects/spring/lib/spring/server.rb:9:in `boot'
	from -e:1:in `<main>'
/Users/senny/Projects/spring/lib/spring/client/run.rb:56:in `call': undefined method `chomp' for nil:NilClass (NoMethodError)
	from /Users/senny/Projects/spring/lib/spring/client/command.rb:7:in `call'
	from /Users/senny/Projects/spring/lib/spring/client.rb:15:in `run'
	from ../../bin/spring:4:in `<main>'
```