add distros.conf

new configuration file describing
* distros (e.g. lenny, squeeze, sid)
* partial distros (e.g. squeeze-security (needs squeeze) or
  experimental (needs sid)
* non-default mirrors (archived distros, backports)

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: add class DistroConfig

for reading distros.conf

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-m: use DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: use DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: use DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: Config: drop get_sources_url()/get_packages_url()

this is now handled with more flexibility in DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: add [global] proxy setting

if set, the http_proxy environment variable will be set to this value
by piuparts-master/-slave/-report (and will be propagated to piuparts)
overriding any http_proxy setting

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p: add --proxy option

retaining the http_proxy variable through sudo is difficult

switch from using HTTP_PROXY to http_proxy

Signed-off-by: Andreas Beckmann <anbe@debian.org>
index.tpl: add news about git clone on github
Merge remote-tracking branch 'andreas/feature/proxy' into develop
Mention http://piuparts.debian.org in README.txt prominently.