Wagtailmenus

------------------

* Updated travis/tox test settings to test against Wagtail 1.9 & Django 1.10.
* Removed a couple of less useful travis/tox environment tests to help with
test speed.
* Made use of 'extras_require' in setup.py to replace multiple requirements
files.
* Optimised the app_settings module so that we can ditch the questionably stuff
we're doing with global value manipulation on app load (solution inspired by
django-allauth).
* Added new symantic version handling to the project (solution inspired by
wagtail)

------------------

* Utilise Django's 'django.template.loader.select_template()' method
to provide a more intuitive way for developers to override templates for
specific menus without having to explicitly specify alternative templates
via settings or via the `template` and `sub_menu_template` options for
each menu tag. See the updated documentation for each tag for information
about where wagtailmenus looks for templates.
* Added the `WAGTAILMENUS_SITE_SPECIFIC_TEMPLATE_DIRS` setting to allow
developers to choose to have wagtailmenus look in additional site-specific
locations for templates to render menus.
* Moved some methods out of `template_tags/menu_tags.py` into a new `utils.py`
file to make `menu_tags.py` easier to read / maintain in future.
* Brazilian Portuguese language translations added by MaxKurama.
* Added try/except to `AbstractMenuItem.relative_url()` so that errors aren't
thrown when `Page.relative_url` returns `None` for some reason.

------------------

* Fixed a bug that would result in {% sub_menu %} being called recursively (
until raising a "maximum recursion depth exceeded" exception) if a
'repeated menu item' was added at anything past the 2nd level. Thanks to
pyMan for raising/investigating.

------------------

* Fixed a bug in the `section_menu` tag when attempting to apply the correct
active class to `section_root` when the `modify_submenu_items()` method has
been overridden to return additional items without an `active_class`
attribute (like in the example code in README).

------------------

* Fixed a bug preventing reordered menu items from retaining their new order
after saving. The Meta class on the new abstract models had knocked out the
`sort_order` ordering from `wagtail.wagtailcore.models.Orderable`.

------------------

* Fixed import error on pip install from version 2.1.0 (Adriaan Tijsseling)