Add __future__ imports to all the files

Also clears a lot of trailing whitespace
Add __future__ imports to binstar_build_client/tests

And clear some trailing whitespace.
Fix some Python 3 issues

This uses six, but that is OK because it already depends on binstar_client,
which depends on six.
Add Python 3 to .binstar.yml
Delete an unused import
Merge pull request #7 from asmeurer/py3

Python 3 support