new preferences handling

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.

As requested by the contribution guide:
https://asciimoo.github.io/searx/dev/contribution_guide.html

Code is refactored and example data are truncated.

Cf. http://legacy.python.org/dev/peps/pep-0008/#blank-lines

add BASE engine in category "Science"

[enh] multilingual autocomplete

implemented for wikipedia, qwant and google

Add Arch Linux Wiki search engine

Content field in Arch Wiki search results is of no real use, more often
than not it contains no usable information and includes too many markup
tags which make the text unreadable. It is safe to remove it.

[fix] duckduckgo's xpaths changed

for some reason installing grunt globally caused travis test to fail.

test_duckduckgo modified to reflect changes in duckduckgo's html

Update link to documentation page