Support for Python 3

Selenium will start supporting Python 3 (experimentally) from the next release (SeleniumHQ/selenium@5cc5085692c6784aefd3f63da85f0c0cb629714a), so I decided to add Python 3 support to webdriverplus as well.

Tests are succeeding, however they cannot be run against the "master"-branch of the selenium repo using travis-ci, because the repo cannot be installed using pip. I've added the next selenium release (2.32.0) to the .travis.yml, so this should only be merged after they release.