Fix /redirect to return absolute URL

This should fix #144 

I also added simple tests to `redirect`,`relative-redirect` and `absolute-redirect`
I wonder if you could write a helper function to make this (and the functions below) less redundant

```python
def _redirect(kind, n, external):
    return redirect(url_for('{0}_redirect_n_times'.format(kind), n=n - 1, _external))
```

Then you could just do:

```python
return _redirect('absolute', n, True)
# or
return _redirect('relative', n, False)
```