Put release archives on a diet

[pombredanne]

Description

The 3.1.1 RC2 release archives are about 180MB. The reasons:

1. a plugins directory that should not be included and should be moved outside the repo
2. two versions of each native third-party: one for Python 2 and one for Python 3

We should go on a diet and could consider:

1. do not include the plugins
2. create differentiated smaller archives for each OS and Python version (e.g. six archives) that would be much smaller

[pombredanne]

This has been implemented. We now have smaller archives, one for each OS