[enh] using yahoo engine instead of xpath for yahoo searches

    content_xpath : //div[@class="definition"]

  - name : yahoo

    engine           : xpath

    search_url       : http://search.yahoo.com/search?p={query}

    results_xpath    : //div[@class="res"]

    url_xpath        : .//h3/a/@href

    title_xpath      : .//h3/a

    content_xpath    : .//div[@class="abstr"]

    suggestion_xpath : //div[@id="satat"]//a

    engine : yahoo

  - name : youtube

    engine : youtube