make fabfile.py use requirements.txt

The '$(<FILE)' syntax expands to a single line, separating the lines of FILE by
spaces. This works on recent shells (means: not dash, busybox sh).
For more backward compability one could use '$(cat FILE)' or even '`cat FILE`'.

The new requirement.txt includes all optional dependencies (taken straight from
fabfile.py) and misses gunicorn and psycopgs2, as those are installed
separately by fabric.
(Better getting fast 90% of packages and some superfluous ones than none at
all)