Convert to python 3

[Anaphory]

Running 2to3 and replacing string raises with ValueError raises makes the basic code useable under python3. (As in, it works fine with my use case afterwards, but I don't use features like graphviz output or SQL yet, so I cannot confirm that it will be fine for those.)

[nnewey]

Thanks Anaphory. I will most likely merge this as a separate Python3 branch, keeping the master branch at 2 for now, as most people still use 2.

[Anaphory]

If you just merged the raise "string" differences, 2to3 should run through without problems, and you don't need to keep two branches up to date, I guess. Sorry I did not think of that earlier, otherwise I would have made two separate commits.

[prashantserai]

Cant see any Python3 branch.. Dropped the plan ?