Generate random trees with management command

Example usage:
```
$ python manage.py random_trees --instance 1 -n 1000 -r 10000
```

Help:

```
$ python manage.py random_trees -h
Usage: manage.py random_trees [options]

Options:
  -v VERBOSITY, --verbosity=VERBOSITY
                        Verbosity level; 0=minimal output, 1=normal output,
                        2=verbose output, 3=very verbose output
  --settings=SETTINGS   The Python path to a settings module, e.g.
                        "myproject.settings.main". If this isn't provided, the
                        DJANGO_SETTINGS_MODULE environment variable will be
                        used.
  --pythonpath=PYTHONPATH
                        A directory to add to the Python path, e.g.
                        "/home/djangoprojects/myproject".
  --traceback           Print traceback on exception
  -i INSTANCE, --instance=INSTANCE
                        Specify the instance to add trees to
  -r RADIUS, --radius=RADIUS
                        Number of meters from the center
  -n N, --number-of-trees=N
                        Number of trees to create
  -d, --delete          Delete previous trees/plots in the instance first
  -p PTREE, --prob-of-tree=PTREE
                        Probability that a given plot will have a tree (0-100)
  --version             show program's version number and exit
  -h, --help            show this help message and exit
```