Update admin.py

Fix bug: changing sort order of top level tree items (tree items without a parent).
Two new item aliases:
* this-parent-sibligs : get items under grandparent.
* this-parent-level-# : get items under ancestor number # from this to top.
Merge pull request #98 from sanjaz/patch-1

Fixed top level tree items sorting.
AUTHORS file is updated
setup.py cleaned.
Tree item dropdown experimentally exposed  (see #84).
Merge branch 'master' of github.com:idlesign/django-sitetree
Merge pull request #99 from bbva-itadmcenter/this-ancestor-siblings

Alias `this-parent-sibligs` is added.
Removed `this-parent-level-` aliases support (see #99).
AUTHORS updated.
Added notice into docs on `sitetreeload` (closes #86).
Version number is bumped up.
CHANGELOG is updated.
Docs updated (see #100).
Added Spanish translation (closes #101).
pypip.in added to README
Adds support for sitetree_page_hint tag
Merge pull request #104 from 7wonders/master

Implemented `sitetree_page_hint` template tag (closes #103).
AUTHORS updated.
runtests formatting changes.
Added experimental support for user-defined sitetree models (closes #64).