Port project to Python 3, drop Python 2 support

[JustinTArthur]

[not ready to merge] Started porting this project to Python 3 several months ago with the goal of taking advantage of new Python features that could greatly improve performance (like asyncio). Finding out about @kyuupichan's ElectrumX project has been a shot to the gut, emotionally. I don't think I have it in me to continue with this effort. If had known about that project when I started, I would have focused on contributing to ElectrumX instead, as it has the same goals as I did for this.

I rebased the changes on recent commits to master and have made this pull request in case someone would like to continue where I left off. This contains no performance improvements as it was intended to achieve a baseline as similar as possible to the Python 2 codebase before moving forward. It may have bugs on the storage side as not all of the operations have been fully tested. @bauerj set up a nice Travis CI if someone would like to add to the test suite.

If no one wants to continue this, feel free to close the pull request un-merged.

[ecdsa]

Sorry to hear that. Still, thank you for your effort. I must say that I am very happy with ElectrumX, because it reduces my workload; I did not have much time to work on the server over the last year.

History pruning is missing in ElectrumX, though; I think in the long run someone will have to implement it.