Update install.rst

I noticed on the website that the documentation regarding Debian was aging, so I took the liberty of adding some new content and making modifications to the source here. I used the existing content as a template to expand the Debian installation section to account for the recent changes.

Debian "wheezy" was released in May 2013 moving "squeeze" to "oldstable" status.

I hope this helps.
Fix to read the settings module specified by --settings or django_settings parameter
try to use cwd() + gunicorn.conf.py as default config if no file is specified . see #52
[gunicorn] added ```gunicorn.conf.py``` as default in case of no -c is specified
prevent call get_default_config_file if args.config exists
Added coverage for get_default_config_file, also a test for default config overrides
Merge pull request #552 from niedbalski/master
Add niedbalski to THANKS
give the initial global_conf to loadcontext to initialize the context properly.
add Moriyoshi Koizumi to THANKS
fix syslog in 2.6

socktype is only supported in 2,7 and sup.

fix #541
Merge pull request #551 from gauravkumar37/master

Fix to read the settings module specified while running via gunicorn command
add CONTRIBUTING doc

fix #473
fix doc abput python 3 support
fix "Expect: continue-100" support on python 3

only bytes should be passed to the socket.

fix #560
add signals doc

add a brief description of the signals handled by Gunicorn.

fix #319
quick doc fixes
fix signals doc file extension
fix link syntax
fix non django application loading

fix error introduced in 423e203db944ea5b250e2816cbeb9c4741cc736f.