Update deb packaging to launch its own supervisord

This pull requests updates the Debian packaging to launch its own supervisord instead of running off of the system supervisor configuration. Supervisor is now launched using the config specified in ```/etc/dd-agent/supervisor_ddagent.conf```. The previous config file located in ```/etc/supervisor/conf.d/ddagent.conf``` is removed, as the default system supervisor config will automatically load the file if it's present

I've tested it on Ubuntu 10.04, 12.04, and Debian 6.  I installed the new version by downloading and modifying the install script to run ```dpkg -i <built_deb>``` instead of ```sude apt-get install datadog-agent{-base}```.

Here are the test cases I ran - let me know if there are any I've missed.
- Fresh install
- Fresh install base
- Install over previous version
- Install base over base previous version
- Install over previous version with modified config file (apt-get prompts the user to choose whether they want to overwrite the config file during install, they should choose yes)
- Remove when agent is running
- Remove base when agent is running
- Remove when agent is not running
- Remove when agent base is not running.