Sitemaps for SITEURL with path elements not generated properly

Pelican is able to be used with a SITEURL containing a path after the domain:

```python
SITEURL = 'http://example.com/path'
```

If you have a setup like this, the sitemaps are not generated properly. This comes from _urljoin_ function used in the code, that strips off path elements without trailing slash.
Adding a trailing slash to SITEURL is also not an option, because this is stripped off by Pelican.

I added a check at the point, where SITEURL is read from Pelican settings. If the value does not end with a slash, it is simply added.
This is safe for SITEURL values with and without path elements.