Add `spring stop` command.

Add Spring::Env#server_running?
Add Spring::Env#pid.

Add the following assertions to acceptance test:

server_running?           -> true|false
assert_server_running     -> "The server should be running but it isn't"
assert_server_not_running -> "The server should not be running but it is"
Merge pull request #42 from rjackson/add_stop_command

Add stop command.