Fixed basejoin. There were two main problems:

Fixed basejoin.  There were two main problems:
- basejoin('some/url', '#name') would strip the last component of
  some/url and resturn some/#name.
- basejoin('file.html', 'relative/path') would return something like
  file:/relative/path, making a relative path into an absolute one.
These bugs are fixed by some drastic changes.  No scheme is added when
none is present (i.e. it works as replacement for posix.joinpath).
If a scheme is present in the second argument, it is returned
unprocessed.  No hostname are added in this case.  If no scheme is
present, the scheme of the first argument, if present, is used.
The algorithm is commented profusely.
Also fixed a typo in a comment.