Debian squeeze enabled packaging

Hi,

As seen in #147 thumbor can run on Debian Squeeze (via PIP) but is hard to package and install as service.

This permit to package it on squeeze as a sysvinit script. letting running as upstart config on ubuntu. (init/upstart file is chosen during the build according to the system running the build)

the last commit is not requisite to build, install and run on Squeeze but permit a generic package.