Add 'make virtualenv' target

This PR adds the command *make virtualenv* that generates (in the *./mp-virtualenv* directory) a virtual environment with all dependencies and mailpile itself installed.

Usage example
```sh
$ make virtualenv
$ source mp-virtualenv/bin/activate
$ mailpile #Shows the mailpile command prompt
```
This allows easy, sandboxed usage and could be useful for deployment in the future.

In order to get this working I needed to add *mailpile.plugins* to *packages* in *setup.py*. I'm not sure if this causes any unintentional side-effects.

This pull request (= the patch represented by this pull request) is hereby released into the public domain. Therefore I'm fine with the licensing described here: https://github.com/pagekite/Mailpile/pull/84#issuecomment-23324989