Feature multi process as deb service

Hello,

As discussed in #144 here is my proposition for multiple instances of Thumbor when starting the service provided by deb package.

I am not really comfortable with upstart so chose to go old fashion way with init. 

BTW the script can so also be use to start processes on other platform than ubuntu/debian/fedora

I invert the behavior of $enabled as "must be 0 to be disabled" instead of "must be 1 to be enabled" so that one can forget it without problem.

Deb package depend on python-crypto package but does not check if apt version available is not offending the on specified in setup.py so for my tests purpose I remove upper limit and didn't notice any problem.

Cheers,

-- 
Damien