try a setuptools/virtualenv -centric approach
rename command to "safe_develop"
gitignore petmail.egg-info/ , now that we're using setuptools
bin/petmail: fix delegate-to-subprocess, need sys.argv too
tolerate pynacl API change: exceptions live elsewhere
update README and travis rules to match new build process
setup.py: don't require setuptools for the initial 'safe_develop' command

We're building a virtualenv anyways, which includes setuptools (or
rather 'distribute'), so it will be available by the time 'safe_develop'
turns around and calls 'develop'.

This change should make it possible to run 'safe_develop' on a system
without setuptools installed.