Python 3

Tests now pass in python `2.7`, `3.3`, and `3.4`. This just requiring fixing some relative imports.

Added `tox.ini`, so you can run `tox` to test in all supported versions.

Added `circle.yml` so the project can be built by [CircleCI](https://circleci.com/) (see the builds of our fork [here](https://circleci.com/gh/cardforcoin/blockr-python)).