add generated autoproj_bootstrap script

Relying on rock-robotics to serve an up-to-date version really
does not work so well (it becomes dependent on the build server
finishing the build). Add it here in the stable branch
documentation update
gitorious: allow specifying the push and pull methods separately
gitorious: make http,ssh the default access method for github

It seems that github prefers that we use https as the pull access
method, so do so. But for pushing, prefer ssh as it does not
require a username and password.
bin: make running aup in the autoproj root the equivalent of aup --all

This is really what people expect. The big difference is that aup --all
will update the osdeps while aup . would not.
osdep: warn about osdep overriding each other only for this specific OS

This avoids having a warning if a package set has for instance added
the relevant osdeps for new operating systems that are not supported
by the original package set.
Merge remote-tracking branches 'origin/make_aup_in_root_equivalent_of_aup_all', 'origin/restrict_osdep_overriding_warning_to_current_os' and 'origin/tune_github_access'
fix make CmdLine.setup_all_packages_directories when the imported root has packages than the local one does not
define PackageSet#each_imported_set
fix display_configuration now that PackageSet#imported_from is a Set
add shortcut to bootstrap from an existing autoproj root

Just do

  autoproj bootstrap git URL --reuse

once the env.sh from the other root has been sourced.
fix Autoproj.in_package_set

The block was not forwarded