Improved logging and dead processes detection

Hello,

I came across foreman trying to find a simple way to run multiple processes inside docker containers with the same guarantee of Foreman that once one of the Procfile defined processes exits (or is killed), Foreman will kill these processes. I added this functionality and improved logging in a way that resembles that of Foreman itself.

I added some test and they all seem to pass in Fedora 20 and my Mac. Let me know what you think.

Thanks for your work in Shoreman, it's been very helpful to me.
The tests seem to be failing on my mac around these lines. It appears BSD `wc(1)` doesn't have a `-L` option :(

```
  it_runs_simple_processes:                        [FAIL]
     bash ./shoreman.sh test/fixtures/simple_procfile
    wc: illegal option -- L
    usage: wc [-clmw] [file ...]
    + rc=1
    + set +x
    + output='shoreman | Started with pid 4396'
    + echo 'shoreman | Started with pid 4396'
    + grep -q Hello
    exit code 1
```