Add help output to Spring::Client::Status.
Show distance ago that processes started in title

Example:

$ bin/spring status
Spring is running:

25233 spring server | rails-3-2 | started 51 secs ago
25331 spring app    | rails-3-2 | started 43 secs ago | development mode
Update README examples
Merge pull request #77 from rjackson/add_description_to_status

Add help output to Spring::Client::Status.
Fix logic error
Have one global server instance, rather than per-session

The global instance is still tied to a particular session, so if that
session ends, then the server will die.

See #78.
Allow multiple commands to run at once

E.g. if you have several terminals open and are running two test
commands at once.

Closes #78