Simplifies tox.ini & adds MySQL + Python 3.

Before starting work on #14, I had to simplify the tox config file in order to easily add postgis.

By the way, why don’t you use tox on Travis CI? That would make it far easier to maintain.
If you agree, I’ll make *.travis.yml* basically look like this:

```yaml
language: python
env:
  global:
    - DTC_MYSQL_USERNAME=travis
    - DTC_PG_USERNAME=postgres
install: pip install tox coveralls
script: tox
after_success: coveralls
```