Changing default url to buildbotURL, if it exists.

This sets the default to using the c['buildbotURL'] if it exists. if not, it resorts to the localhost one.
While this is an improvement on the previous version, I believe we can still go further in simplifications: If I get it right, there are multiple layer, ordered by priority:

1. www['url']
2. bbURL
3. localhost/%d
4. ?

Maybe 4 is not necessary if we make sure www['port'] is defined, I didn't checked this.

This ordering should stand out from the source. Something like:

``` py
if url:
    # Maybe warn if bbURL is defined that it is not used
    return    # Or ``pass`` if you dislike return in middle of function
elif bbURL:
    self.www['url'] = bbURL
elif port: # This could become an else if we are sure that port is always defined.
    self.www['url'] = 'http://localhost:%d/' % (port,)
else:
    ???
```