Fix 'set' object does not support indexing

[fix] currency_convert: make unit tests pass

Fixed broken link

[enh] currency_convert: allow strings like google's "convert 1 euro to dollars"

In some circumstances, it is necessary to increase size of pool.

Vim-inspired hotkeys plugin

Oscar improvement

[enh] wikipedia infobox

creates simple multilingual infobox using wikipedia's api

[fix] incorrect URLs in Reddit image search results - closes #538

Add a few search engines

Add doku engine

General minor refactors

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