Pass parsed hostname around explictly instead of storing it on the urlsplit result.

Pass parsed hostname around explictly instead of storing it on the urlsplit result.

In some versions of python (including 2.5 and 3.2 but not 2.6 or 2.7),
urlsplit results use __slots__ and therefore don't allow artibrary attributes
to be set.

Also move the storing of the parsed hostname above hostname_mapping,
since the cert should be checked against the requested url, not the remapped
one.

Closes #448.