use stdeb 0.8.1 (supports python 3 natively)
Build and upload source packages for each Python version.

The desire for this change is to support distributing source packages
alongside the binary packages from the debian repositories. See
http://answers.ros.org/question/187924 for more discussion.

The implementation here is a little suboptimal because of a desire to
minimize required changes in downstream packages. Ideally, a single
source package (per Ubuntu/Debian release) would be generated for all
Python versions. However, doing so would require changes in each
package distributed with this software to merge existing stdeb2.cfg
and stdeb3.cfg files. Furthermore, this would necessitate further
changes in the upload and building logic to deal with cases where
different source packages would need to deal with different
Ubuntu/Debian releases requiring different binary packages. (E.g. a
source package for Ubuntu Precise may want to build a binary only for
Python 2 whereas it may be desired that the Ubuntu Trusy source
package specifies building binaries for Python 2 and 3.) The cost paid
is a duplication of source packages and deviation from standard stdeb
behavior.  Overcoming these issues seems doable but would require
substantial churn in the packages using ros_release_python and
potentially some upstream changes in stdeb.

This requires stdeb >= 0.8.0.
translate current stdeb3.cfg to upstream stdeb.cfg format
bump required stdeb version to 0.8.2

There was a bug in stdeb 0.8.1 which would not actually build python3
packages with python3 if there was no python2 package also being
built.
Merge pull request #7 from astraw/upload-source-packages

Upload source packages