Fix startpage

Fix issue with unicode caracters in startpage : we shouldn't urlencode them if we are using POST.
Should fix #169. @dimqua can you confirm ?

searx/engines/startpage.py

 # do search-request
 def request(query, params):
     offset = (params['pageno'] - 1) * 10
-    query = urlencode({'q': query})[2:]
 
     params['url'] = search_url
     params['method'] = 'POST'