added versioning code and a version #

Per @astrofrog's recommendation, we're using pyregion in aplpy, and it would be nice to be able to explicitly set a version dependency.

This is one way to deal with versioning; there are other options but this one works:

```
$ python -c "import pyregion; print pyregion.__version__"
1.1
```

```
$ python setup.py --version
1.1
```