py3: fix running abzsubmit from current folder

"from . import" doesn't work in this context anymore
since the context is where abzsubmit is, which is not inside the abz
module.
"from abz import" works fine.
py3: introduce compat module

Needed for urlunparse and RawConfigParser.
The module checks if we are running in Python 2 or 3 and uses different
imports for each (renames only).
add more tested python versions to README
py3: decode only when necessary

In Python 2 strings are normally byte strings.
In Python 3 strings are unicode strings by default.

So msg.decode() is not necessary and can't be done in Python 3 since the
string is already a unicode string.
This adds a decode that does nothing when the input is not a byte
string.

Note that encoding can be done the same, but is not necessary here.
py3: fix vendor import

Using `.vendor` works for Python 2 and 3.
Just `vendor` doesn't work for Python 3 because this is relative to the
abz module, not the abzsubmit script.
Merge pull request #22 from JonnyJD/py3

Python 3 support