pep8 is deprecated, it is replaced by pycodestyle.

Move spot project to python 3. The support of python 2 is
removed related to the issue https://github.com/asciimoo/searx/issues/1471.

The update is mainly done with `2to3` tool and a lot of changes for
the unicode support (see https://docs.python.org/3/howto/unicode.html)

Close: #7

this fixes duckduckgo error response

Add match_language function in utils to match any user given
language code with a list of engine's supported languages.

Also add language_aliases dict on each engine to translate
standard language codes into the custom codes used by the engine.

The exact order is
* first from SEARX_SETTINGS_PATH,
* if not found then from searx code base,
* if not found then from /etc/searx/settings.yml
* if not found an exception stops searx loading

[mod] add a __common__ template that can't be selected but that provides a common place for shared templates.

What has been moved into this template :
* opensearch*.xml is always the same whatever the themes.
* the text inside */about.html

Unfortunately, it is quite slow so it is disabled.
Furthermore, the display of number of files is wrong
on digbt.org, so it is not displayed on searx.

occitan, for example

Preferences class was introduced in order to handle user preferences. Right now
it parses cookies and the form in preferences. Also it can retrieve settings
based on the name of the setting.

ATTENTION
Please note that engine preferences are handled differently from now on. So it
introduces incompatible changes. Every user who has saved preferences should reset and
save his/her settings again.

This change was needed, because everytime a default disabled engine was
added saved user preferences would broke. Now engine setting tracking is
fixed.