merge visualization branch

The web server is included as a submodule.
The dependencies for the web (tornado) are not installed by default, but they
can be installed as an extra:

```
pip install soil[web]
```

Once installed, the soil web can be used like this:

```
soil-web

OR

python -m soil.web
```

There are other minor changes:

* History re-connects to the sqlite database if it is used from a different
thread.
* Environment accepts additional parameters (so it can run simulations with
`visualization_params` or any other in the future).
* The simulator class is no longer necessary
* Logging is done in the same thread, and the simulation is run in a separate
one. This had to be done because it was creating some problems with tornado not
being able to find the current thread during logs, which caused hundreds of
repeated lines in the web "console".
* The player is slightly modified in this version. I noticed that when the
  visualization was playing, if you clicked somewhere it would change for a
  second, and then go back to the previous place. The code for the playback
  seemed too complex, especially speed control, so I rewrote some parts. I
  might've introduced new bugs.

Dockerfile

@@ -1,3 +1,5 @@
 FROM python:3.4-onbuild
 
+RUN pip install '.[web]'
+
 ENTRYPOINT ["python", "-m", "soil"]