Make setup.py install work in a fresh virtualenv

[gthb]

i.e. without dependencies already installed.

[smari]

This is a bit of a dirty hack. Can you explain how it is useful? :-)

[gthb]

It fixes the problem that setup.py currently can't install Mailpile's dependencies unless they are already installed. : )

Specifically, this import:

from mailpile.defaults import APPVER

includes importing mailpile, which in turn imports other modules under mailpile, which fails if dependencies are not installed.

Alternative ways to fix this include:

1. maintaining APPVER in two places (in setup.py and in application code) and keeping them in sync manually
2. frobnicating imports so that the module containing the version can be imported without that cascading to importing all the rest
3. (just found this one): maintaining the version only in setup.py, and reading it in the application using pkg_resources. See http://stackoverflow.com/a/17638236/74683

That last one is probably fine (I haven't done it), in which case this pull request can be scrapped.

[smari]

Okay. Will merge this for now, but I think the pkg_resources approach might be better - or having some external file that contains version info...