Auto-generate list of "packages"

When moving code around, creating new subdirs, one had to add/modify packages list
in setup.py.
So, basically, this patch is just generating an up-to-date list on runtime, including
all subdirs under picard/ and converting them to modules-like names (replace path separator by a dot).
Merge pull request #363 from zas/setup_packages_fix

Auto-generate list of "packages"