debian packages: remove pip, add virtualenv

Move pip to a virtualenv... as per discussion with Matti.
Motivation: keep delta between dev and prod small, keep spreadpi images easy to update, avoid permissions problems, ...
install pip from source

Partial cleanup... should at least install the virtualenv so we can clean up the image interactively from here...
Update 10-spreads.sh
further cleanup of 10-spreads.sh

Still contains an error handling subshell for spreads user!
Create install_spreads

This gets executed as user spreads, _not_ as root., from deliver/scripts/10-spreads.sh
Rename install_spreads to install_spreads.sh
split off spreads virtualenv into separate file
fix typo
Update install_spreads.sh
fix typo
add libyaml-dev deb dependency
netifaces --allow-external and --allow-unverified
Update install_spreads.sh
move cython from deb to pip installed
move cython from deb to pip installed
install develop code inside virtualenv dir
remove python-exiv2 debian dependency
Update 10-spreads.sh
Set permissions for install_spreads, call it correctly from installation scripts
Update build.sh