repo: let the user choose/override which upstream repo to use

Also, provides proper defaults, as far as possible. I refrain from
encoding the current matrix of available versions vs. available flavours.
e.g. there are no RCs for Fedora and there's no stable version for wheezy,
but that'll change fast, so I do not see much value in encoding that.
proxy: fix dependencies when installing
settings: add a reminder
Merge pull request #18 from DavidS/master

fix the repository class and improve proxy installation