Upload nightlies in only one folder

[AntoineTurmel]

Currently nightlies are separated in different folders by OS and arch. We should do like Mozilla and POTI and make only one folder for multi OS/arch:

• http://developer.songbirdnest.com/builds/trunk/latest/
• http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-mozilla-central/

so no arch add-ons will be builded once only.

It depends on issue #3