Use types from pudo's 'typecast' library, PEP8 etc

[davidread]

@pudo's #155 with fixes from @StevenMaude

This changeset supersedes #139, and includes a variety of changes to improve Python 3 support (using six directly).

I expect this to fix #154, #146, #50, #40, #117 and make #37, #140, #148, #149 into issues for typecast.

Let's see if we can get this merged?

[pudo]

+1 I'm so keen to finally get a cleanup of messytables.

[fxsman]

Can someone push this fix please?

[davidread]

To resolve the conflicts on this, @pudo should decide whether he wants these new types: #166 in typecast, which is where this PR factors out this functionality.