Fix #141: TranslatableModelAdmin uses absolute URLs for language tabs, breaks reverse proxies

On the language tabs of our admin, behind a reverse proxy, we see this:

    <input type="hidden" class="language_button selected" name="en" />
    <span class="current">English</span>
    <span class="empty"><a href="http://fen-vz-inaspauthoraid.fen.aptivate.org/en/admin/news/newsitem/add/?language=es">Spanish</a> </span>

`fen-vz-inaspauthoraid.fen.aptivate.org` is an internal hostname which cannot
be accessed without a VPN. It's not the URL that we used to load the page,
through the reverse proxy.

It's not necessary to include a host and protocol in the rendered links. You
can save a little bandwidth (page size) and avoid this problem by using
relative links:

    url = '%s?%s' % (request.path, urlencode(get))
Removed all references to Model.objects, replaced by Model._default_manager.
Fixed issue with select_related on proxy models: get_field_by_name result interpretation was incorrect
Added test case for select_related on proxies
Include documentation in source distribution.
Merge pull request #163 from fladi/include_docs

Include documentation in source distribution.
Merge pull request #162 from spectras/select_related_fix

Fix: select_related on proxy models fails: get_field_by_name result interpretation is incorrect
Merge pull request #145 from spectras/defaultmanager_fix

Fix: Removed all references to Model.objects, replaced by _default_manager
Merge pull request #151 from aptivate/ticket_141

This looks good, thanks!