New mail sender and helper classes for init/push

Moved `sendspam.py` to `sendmail.py` and reformatted it as a general-purpose
mail sender. (Currently some other scripts are using their own mailer; it would
be nice if everything worked in one uniform interface.)

The base application class is being rehaled and moved from rcommon.py to
__init__.py so that everyone can use it a little easier. The script-specific
classes should have helper classes that extend this base class - see pushpackage
for more details.