Add Google Scholar search engine

AUTHORS.rst

@@ -51,3 +51,4 @@ generally made searx better:
 - Ashutosh Das @pyprism
 - YuLun Shih @imZack
 - Dmitry Mikhirev @mikhirev
+- David A Roberts `@davidar <https://github.com/davidar>`_

searx/settings.yml

@@ -200,6 +200,20 @@ engines:
     engine : google_news
     shortcut : gon
 
+  - name : google scholar
+    engine : xpath
+    paging : True
+    search_url : https://scholar.google.com/scholar?start={pageno}&q={query}&hl=en&as_sdt=0,5&as_vis=1'
+    results_xpath : //div[@class="gs_r"]/div[@class="gs_ri"]
+    url_xpath : .//h3/a/@href
+    title_xpath : .//h3/a
+    content_xpath : .//div[@class="gs_rs"]
+    suggestion_xpath : //div[@id="gs_qsuggest"]/ul/li
+    page_size : 10
+    first_page_num : 0
+    categories : science
+    shortcut : gos
+
   - name : google play apps
     engine        : xpath
     search_url    : https://play.google.com/store/search?q={query}&c=apps